Hi Stefano,
Vincent gave an update on his virtio-scmi work at the last Stratos sync call and the discussion moved onto next steps. Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
* How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
* How to handle mapping of memory
AIUI the Xen model is the FE guest explicitly makes grant table requests to expose portions of it's memory to other domains. Can the BE query the hypervisor itself to discover the available grants or does it require coordination with Dom0/XenStore for that information to be available to the BE domain?
* How to handle signalling
I guess this requires a minimal implementation of the IOREQ calls for Zephyr so we can register the handler in the backend? Does the IOREQ API allow for a IPI style notifications using the global GIC IRQs?
Forgive the incomplete notes from the Stratos sync, I was trying to type while participating in the discussion so hopefully this email captures what was missed:
https://linaro.atlassian.net/wiki/spaces/STR/pages/28682518685/2022-02-03+Pr...
Vincent, anything to add?
On Mon, 7 Feb 2022, Alex Bennée wrote:
Hi Stefano,
Vincent gave an update on his virtio-scmi work at the last Stratos sync call and the discussion moved onto next steps.
Hi Alex,
I don't know the specifics of virtio-scmi, but if it is about power, clocks, reset, etc. like the original SCMI protocol, then virtio-scmi is likely going to be very different from all the other virtio frontends and backends. That's because SCMI requires a full view of the system, which is different from something like virtio-net that is limited to the emulation of 1 device. For this reason, it is likely that the virtio-scmi backend would be a better fit in Xen itself, rather than run in userspace inside a VM.
FYI, a good and promising approach to handle both SCMI and SCPI is the series recently submitted by EPAM to mediate SCMI and SCPI requests in Xen: https://marc.info/?l=xen-devel&m=163947444032590
(Another "special" virtio backend is virtio-iommu for similar reasons: the guest p2m address mappings and also the IOMMU drivers are in Xen. It is not immediately clear whether a virtio-iommu backend would need to be in Xen or run as a process in dom0/domU.)
On the other hand, for all the other "normal" protocols (e.g. virtio-net, virtio-block, etc.) the backend would naturally run as a process in dom0 or domU (e.g. QEMU in Dom0) as one would expect.
Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
Unfortunately I have a regular conflict which prevents me from being able to join the Stratos calls. However, I can certainly make myself available for one call (unless something unexpected comes up).
- How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
I'll try to answer the question for a generic virtio frontend and backend instead (not SCMI because SCMI is unique due to the reasons above.)
Yes, xenstore is the easiest way to exchange configuration information between domains. I think EPAM used xenstore to exchange the configuration information in their virtio-block demo. There is a way to use xenstore even between dom0less VMs: https://marc.info/?l=xen-devel&m=164340547602391 Not just xenstore but full PV drivers too. However, in the dom0less case xenstore is going to become available some time after boot, not immediately at startup time. That's because you need to wait until xenstored is up and running.
There are other ways to send data from one VM to another which are available immediately at boot, such as Argo and static shared memory.
But dom0less is all about static partitioning, so it makes sense to exploit the build-time tools to the fullest. In the dom0less case, we already know what is going to run on the target before it is even turned on. As an example, we might have already prepared an environment with 3 VMs using Yocto and ImageBuilder. We could also generate all configurations needed and place them inside each VMs using Yocto's standard tools and ImageBuilder. So for dom0less, I recommend to go via a different route and pre-generate the configuration directly where needed instead of doing dynamic discovery.
- How to handle mapping of memory
AIUI the Xen model is the FE guest explicitly makes grant table requests to expose portions of it's memory to other domains. Can the BE query the hypervisor itself to discover the available grants or does it require coordination with Dom0/XenStore for that information to be available to the BE domain?
Typically the frontend passes grant table references to the backend (i.e. instead of plain guest physical addresses on the virtio ring.) Then, the backend maps the grants; Xen checks that the mapping is allowed.
We might be able to use the same model with virtio devices. A special pseudo-IOMMU driver in Linux would return a grant table reference and an offset as "DMA address". The "DMA address" is passed to the virtio backend over the virtio ring. The backend would map the grant table reference using the regular grant table hypercalls.
- How to handle signalling
I guess this requires a minimal implementation of the IOREQ calls for Zephyr so we can register the handler in the backend? Does the IOREQ API allow for a IPI style notifications using the global GIC IRQs?
Forgive the incomplete notes from the Stratos sync, I was trying to type while participating in the discussion so hopefully this email captures what was missed:
https://linaro.atlassian.net/wiki/spaces/STR/pages/28682518685/2022-02-03+Pr...
Yes, any emulation backend (including virtio backends) would require an IOREQ implementation, which includes notifications via event channels. Event channels are delivered as a GIC PPI interrupt to the Linux kernel. Then, the kernel sends the notification to userspace via a file descriptor.
Stefano Stabellini stefano.stabellini@xilinx.com writes:
On Mon, 7 Feb 2022, Alex Bennée wrote:
Hi Stefano,
Vincent gave an update on his virtio-scmi work at the last Stratos sync call and the discussion moved onto next steps.
Hi Alex,
I don't know the specifics of virtio-scmi, but if it is about power, clocks, reset, etc. like the original SCMI protocol, then virtio-scmi is likely going to be very different from all the other virtio frontends and backends. That's because SCMI requires a full view of the system, which is different from something like virtio-net that is limited to the emulation of 1 device. For this reason, it is likely that the virtio-scmi backend would be a better fit in Xen itself, rather than run in userspace inside a VM.
That may be a good solution for Xen but I still think it's worthwhile being able to package SCMI in a VM for other hypervisors. We are just happening to use Xen as a nice type-1 example.
Vincents SCMI server code is portable anyway and can reside in a Zephyr app, firmware blob or a userspace vhost-user client.
FYI, a good and promising approach to handle both SCMI and SCPI is the series recently submitted by EPAM to mediate SCMI and SCPI requests in Xen: https://marc.info/?l=xen-devel&m=163947444032590
(Another "special" virtio backend is virtio-iommu for similar reasons: the guest p2m address mappings and also the IOMMU drivers are in Xen. It is not immediately clear whether a virtio-iommu backend would need to be in Xen or run as a process in dom0/domU.)
On the other hand, for all the other "normal" protocols (e.g. virtio-net, virtio-block, etc.) the backend would naturally run as a process in dom0 or domU (e.g. QEMU in Dom0) as one would expect.
Can domU's not be given particular access to HW they might want to tweak? I assume at some point a block device backend needs to actually talk to real HW to store the blocks (even if in most cases it would be a kernel doing the HW access on it's behalf).
Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
Unfortunately I have a regular conflict which prevents me from being able to join the Stratos calls. However, I can certainly make myself available for one call (unless something unexpected comes up).
- How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
I'll try to answer the question for a generic virtio frontend and backend instead (not SCMI because SCMI is unique due to the reasons above.)
Yes, xenstore is the easiest way to exchange configuration information between domains. I think EPAM used xenstore to exchange the configuration information in their virtio-block demo. There is a way to use xenstore even between dom0less VMs: https://marc.info/?l=xen-devel&m=164340547602391 Not just xenstore but full PV drivers too. However, in the dom0less case xenstore is going to become available some time after boot, not immediately at startup time. That's because you need to wait until xenstored is up and running.
There are other ways to send data from one VM to another which are available immediately at boot, such as Argo and static shared memory.
But dom0less is all about static partitioning, so it makes sense to exploit the build-time tools to the fullest. In the dom0less case, we already know what is going to run on the target before it is even turned on. As an example, we might have already prepared an environment with 3 VMs using Yocto and ImageBuilder. We could also generate all configurations needed and place them inside each VMs using Yocto's standard tools and ImageBuilder. So for dom0less, I recommend to go via a different route and pre-generate the configuration directly where needed instead of doing dynamic discovery.
Even in a full dom0less setup you still need to manage lifetimes somehow if a guest reboots.
- How to handle mapping of memory
AIUI the Xen model is the FE guest explicitly makes grant table requests to expose portions of it's memory to other domains. Can the BE query the hypervisor itself to discover the available grants or does it require coordination with Dom0/XenStore for that information to be available to the BE domain?
Typically the frontend passes grant table references to the backend (i.e. instead of plain guest physical addresses on the virtio ring.) Then, the backend maps the grants; Xen checks that the mapping is allowed.
We might be able to use the same model with virtio devices. A special pseudo-IOMMU driver in Linux would return a grant table reference and an offset as "DMA address". The "DMA address" is passed to the virtio backend over the virtio ring. The backend would map the grant table reference using the regular grant table hypercalls.
- How to handle signalling
I guess this requires a minimal implementation of the IOREQ calls for Zephyr so we can register the handler in the backend? Does the IOREQ API allow for a IPI style notifications using the global GIC IRQs?
Forgive the incomplete notes from the Stratos sync, I was trying to type while participating in the discussion so hopefully this email captures what was missed:
https://linaro.atlassian.net/wiki/spaces/STR/pages/28682518685/2022-02-03+Pr...
Yes, any emulation backend (including virtio backends) would require an IOREQ implementation, which includes notifications via event channels. Event channels are delivered as a GIC PPI interrupt to the Linux kernel. Then, the kernel sends the notification to userspace via a file descriptor.
Thanks.
On Fri, 11 Feb 2022, Alex Bennée wrote:
FYI, a good and promising approach to handle both SCMI and SCPI is the series recently submitted by EPAM to mediate SCMI and SCPI requests in Xen: https://marc.info/?l=xen-devel&m=163947444032590
(Another "special" virtio backend is virtio-iommu for similar reasons: the guest p2m address mappings and also the IOMMU drivers are in Xen. It is not immediately clear whether a virtio-iommu backend would need to be in Xen or run as a process in dom0/domU.)
On the other hand, for all the other "normal" protocols (e.g. virtio-net, virtio-block, etc.) the backend would naturally run as a process in dom0 or domU (e.g. QEMU in Dom0) as one would expect.
Can domU's not be given particular access to HW they might want to tweak? I assume at some point a block device backend needs to actually talk to real HW to store the blocks (even if in most cases it would be a kernel doing the HW access on it's behalf).
Yes, it would. Block and network are subsystems with limited visibility, access, and harmful capabilities (assuming IOMMU).
If the block device goes down or is misused, block might not work but everything else is expected to work. Block only requires visibility of the block device for it to work. The same is true for network, GPU, USB, etc.
SCMI is different. If SCMI is misused the whole platform is affected. SCMI implies visibility of everything in the system. It is not much about emulating SCMI but more about mediating SCMI calls.
In other words, SCMI is not a device, it is a core interface. In a Xen model, Xen virtualizes CPU and memory and other core features/interfaces (timers, interrupt controller, IOMMU, etc). The PCI root complex is handled by Xen too. Individual (PCI and non-PCI) devices are assigned to guests.
These are the reasons why I think the best way to enable SCMI in upstream Xen is with a mediator in the hypervisor as it is currently in development. Any chances you could combine your efforts with EPAM's outstanding series? You might be able to spot gaps if any, and might even have already code to fill those gaps. It would be fantastic to have your reviews and/or contributions on xen-devel.
Otherwise, if you have to run the virtio-scmi backend in userspace, why not try to get it to work on Xen :-) It might not be the ideal solution, but it could be a good learning experience and pave the way for the other virtio backends which definitely will be in userspace (virtio-block, virtio-gpu, etc).
Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
Unfortunately I have a regular conflict which prevents me from being able to join the Stratos calls. However, I can certainly make myself available for one call (unless something unexpected comes up).
- How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
I'll try to answer the question for a generic virtio frontend and backend instead (not SCMI because SCMI is unique due to the reasons above.)
Yes, xenstore is the easiest way to exchange configuration information between domains. I think EPAM used xenstore to exchange the configuration information in their virtio-block demo. There is a way to use xenstore even between dom0less VMs: https://marc.info/?l=xen-devel&m=164340547602391 Not just xenstore but full PV drivers too. However, in the dom0less case xenstore is going to become available some time after boot, not immediately at startup time. That's because you need to wait until xenstored is up and running.
There are other ways to send data from one VM to another which are available immediately at boot, such as Argo and static shared memory.
But dom0less is all about static partitioning, so it makes sense to exploit the build-time tools to the fullest. In the dom0less case, we already know what is going to run on the target before it is even turned on. As an example, we might have already prepared an environment with 3 VMs using Yocto and ImageBuilder. We could also generate all configurations needed and place them inside each VMs using Yocto's standard tools and ImageBuilder. So for dom0less, I recommend to go via a different route and pre-generate the configuration directly where needed instead of doing dynamic discovery.
Even in a full dom0less setup you still need to manage lifetimes somehow if a guest reboots.
Sure but that's not a problem: all the info and configuration related to rebooting the guest can also be pre-generated in Yocto or ImageBuilder.
As an example, it is already possible (although rudimental) in ImageBuilder to generate the dom0less configuration and also the domU xl config file for the same domU with passthrough devices.
On Sat, 12 Feb 2022 at 00:34, Stefano Stabellini stefano.stabellini@xilinx.com wrote:
On Fri, 11 Feb 2022, Alex Bennée wrote:
FYI, a good and promising approach to handle both SCMI and SCPI is the series recently submitted by EPAM to mediate SCMI and SCPI requests in Xen: https://marc.info/?l=xen-devel&m=163947444032590
(Another "special" virtio backend is virtio-iommu for similar reasons: the guest p2m address mappings and also the IOMMU drivers are in Xen. It is not immediately clear whether a virtio-iommu backend would need to be in Xen or run as a process in dom0/domU.)
On the other hand, for all the other "normal" protocols (e.g. virtio-net, virtio-block, etc.) the backend would naturally run as a process in dom0 or domU (e.g. QEMU in Dom0) as one would expect.
Can domU's not be given particular access to HW they might want to tweak? I assume at some point a block device backend needs to actually talk to real HW to store the blocks (even if in most cases it would be a kernel doing the HW access on it's behalf).
Yes, it would. Block and network are subsystems with limited visibility, access, and harmful capabilities (assuming IOMMU).
If the block device goes down or is misused, block might not work but everything else is expected to work. Block only requires visibility of the block device for it to work. The same is true for network, GPU, USB, etc.
SCMI is different. If SCMI is misused the whole platform is affected. SCMI implies visibility of everything in the system. It is not much about emulating SCMI but more about mediating SCMI calls.
In other words, SCMI is not a device, it is a core interface. In a Xen model, Xen virtualizes CPU and memory and other core features/interfaces (timers, interrupt controller, IOMMU, etc). The PCI root complex is handled by Xen too. Individual (PCI and non-PCI) devices are assigned to guests.
These are the reasons why I think the best way to enable SCMI in upstream Xen is with a mediator in the hypervisor as it is currently in development. Any chances you could combine your efforts with EPAM's outstanding series? You might be able to spot gaps if any, and might even have already code to fill those gaps. It would be fantastic to have your reviews and/or contributions on xen-devel.
Otherwise, if you have to run the virtio-scmi backend in userspace, why
Just to clarify, this goal is not to run the scmi backend as a linux userspace app but to run a virtual power coprocessor that will handle everything which is not system critical and will change from one product to another which make it quite hard to maintain in the hypervisor.
I have only looked at the cover letter which mentions the use of SMC call which will be trapped by Xen before being modified and forward to ATF. AFAICT, the ATF execution context is quite simple and synchronous with the request. In our case, we want to be able to manage to I2C device as an example or to notifies VMs with aynshorous event like sensor or performance change which virtio-scmi support
not try to get it to work on Xen :-) It might not be the ideal solution, but it could be a good learning experience and pave the way for the other virtio backends which definitely will be in userspace (virtio-block, virtio-gpu, etc).
Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
Unfortunately I have a regular conflict which prevents me from being able to join the Stratos calls. However, I can certainly make myself available for one call (unless something unexpected comes up).
- How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
I'll try to answer the question for a generic virtio frontend and backend instead (not SCMI because SCMI is unique due to the reasons above.)
Yes, xenstore is the easiest way to exchange configuration information between domains. I think EPAM used xenstore to exchange the configuration information in their virtio-block demo. There is a way to use xenstore even between dom0less VMs: https://marc.info/?l=xen-devel&m=164340547602391 Not just xenstore but full PV drivers too. However, in the dom0less case xenstore is going to become available some time after boot, not immediately at startup time. That's because you need to wait until xenstored is up and running.
There are other ways to send data from one VM to another which are available immediately at boot, such as Argo and static shared memory.
But dom0less is all about static partitioning, so it makes sense to exploit the build-time tools to the fullest. In the dom0less case, we already know what is going to run on the target before it is even turned on. As an example, we might have already prepared an environment with 3 VMs using Yocto and ImageBuilder. We could also generate all configurations needed and place them inside each VMs using Yocto's standard tools and ImageBuilder. So for dom0less, I recommend to go via a different route and pre-generate the configuration directly where needed instead of doing dynamic discovery.
Even in a full dom0less setup you still need to manage lifetimes somehow if a guest reboots.
Sure but that's not a problem: all the info and configuration related to rebooting the guest can also be pre-generated in Yocto or ImageBuilder.
As an example, it is already possible (although rudimental) in ImageBuilder to generate the dom0less configuration and also the domU xl config file for the same domU with passthrough devices.
Hi Stefano,
On Tue, 8 Feb 2022 at 01:16, Stefano Stabellini stefano.stabellini@xilinx.com wrote:
On Mon, 7 Feb 2022, Alex Bennée wrote:
Hi Stefano,
Vincent gave an update on his virtio-scmi work at the last Stratos sync call and the discussion moved onto next steps.
Hi Alex,
I don't know the specifics of virtio-scmi, but if it is about power, clocks, reset, etc. like the original SCMI protocol, then virtio-scmi is
virtio-scmi is one transport channel that support SCMI protocol
likely going to be very different from all the other virtio frontends
The virtio-scmi front-end is merged mainline
and backends. That's because SCMI requires a full view of the system, which is different from something like virtio-net that is limited to the emulation of 1 device. For this reason, it is likely that the virtio-scmi backend would be a better fit in Xen itself, rather than run in userspace inside a VM.
Not sure what you mean when you say that SCMI requires a full view of the system. If you are referring to the system wide resources which reset or power up/down the whole SoC, this is not really what we are targeting here. Those system wide resources should already be handled by a dedicated power coprocessor. In our case, the IPs of the SoC will be handled by different VMs but those IPs are usually sharing common resources like a parent PLL , a power domain or a clock gating reg as few examples. Because all those VMs can't directly set these resources without taking into account others and because the power coprocessor doesn't have an unlimited number of channels, we add an SCMI backend that will gather and proxy the VM request before accessing the register that gates some clocks IP as an example or before powering down an external regulator shared between the camera and another device. This SCMI backend will most probably also send request with OSPM permission access to the power coprocessor once aggregating all the VMs ' request We are using virtio-cmi protocol because it has the main advantage of not being tied to an hypervisor
In our PoC, the SCMI backend is running with zehyr and reuse the same software that can run in the power coprocessor which helps splitting what is critical and must be handled by power coprocessor and what is not critical for the system (what is usually managed by linux directly when their no hypervisor involved typically)
FYI, a good and promising approach to handle both SCMI and SCPI is the series recently submitted by EPAM to mediate SCMI and SCPI requests in Xen: https://marc.info/?l=xen-devel&m=163947444032590
(Another "special" virtio backend is virtio-iommu for similar reasons: the guest p2m address mappings and also the IOMMU drivers are in Xen. It is not immediately clear whether a virtio-iommu backend would need to be in Xen or run as a process in dom0/domU.)
On the other hand, for all the other "normal" protocols (e.g. virtio-net, virtio-block, etc.) the backend would naturally run as a process in dom0 or domU (e.g. QEMU in Dom0) as one would expect.
Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
Unfortunately I have a regular conflict which prevents me from being able to join the Stratos calls. However, I can certainly make myself available for one call (unless something unexpected comes up).
- How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
I'll try to answer the question for a generic virtio frontend and backend instead (not SCMI because SCMI is unique due to the reasons above.)
Yes, xenstore is the easiest way to exchange configuration information between domains. I think EPAM used xenstore to exchange the configuration information in their virtio-block demo. There is a way to use xenstore even between dom0less VMs: https://marc.info/?l=xen-devel&m=164340547602391 Not just xenstore but full PV drivers too. However, in the dom0less case xenstore is going to become available some time after boot, not immediately at startup time. That's because you need to wait until xenstored is up and running.
There are other ways to send data from one VM to another which are available immediately at boot, such as Argo and static shared memory.
But dom0less is all about static partitioning, so it makes sense to exploit the build-time tools to the fullest. In the dom0less case, we already know what is going to run on the target before it is even turned on. As an example, we might have already prepared an environment with 3 VMs using Yocto and ImageBuilder. We could also generate all configurations needed and place them inside each VMs using Yocto's standard tools and ImageBuilder. So for dom0less, I recommend to go via a different route and pre-generate the configuration directly where needed instead of doing dynamic discovery.
- How to handle mapping of memory
AIUI the Xen model is the FE guest explicitly makes grant table requests to expose portions of it's memory to other domains. Can the BE query the hypervisor itself to discover the available grants or does it require coordination with Dom0/XenStore for that information to be available to the BE domain?
Typically the frontend passes grant table references to the backend (i.e. instead of plain guest physical addresses on the virtio ring.) Then, the backend maps the grants; Xen checks that the mapping is allowed.
We might be able to use the same model with virtio devices. A special pseudo-IOMMU driver in Linux would return a grant table reference and an offset as "DMA address". The "DMA address" is passed to the virtio backend over the virtio ring. The backend would map the grant table reference using the regular grant table hypercalls.
- How to handle signalling
I guess this requires a minimal implementation of the IOREQ calls for Zephyr so we can register the handler in the backend? Does the IOREQ API allow for a IPI style notifications using the global GIC IRQs?
Forgive the incomplete notes from the Stratos sync, I was trying to type while participating in the discussion so hopefully this email captures what was missed:
https://linaro.atlassian.net/wiki/spaces/STR/pages/28682518685/2022-02-03+Pr...
Yes, any emulation backend (including virtio backends) would require an IOREQ implementation, which includes notifications via event channels. Event channels are delivered as a GIC PPI interrupt to the Linux kernel. Then, the kernel sends the notification to userspace via a file descriptor.
On Tue, 15 Feb 2022, Vincent Guittot wrote:
On Tue, 8 Feb 2022 at 01:16, Stefano Stabellini stefano.stabellini@xilinx.com wrote:
On Mon, 7 Feb 2022, Alex Bennée wrote:
Hi Stefano,
Vincent gave an update on his virtio-scmi work at the last Stratos sync call and the discussion moved onto next steps.
Hi Alex,
I don't know the specifics of virtio-scmi, but if it is about power, clocks, reset, etc. like the original SCMI protocol, then virtio-scmi is
virtio-scmi is one transport channel that support SCMI protocol
likely going to be very different from all the other virtio frontends
The virtio-scmi front-end is merged mainline
and backends. That's because SCMI requires a full view of the system, which is different from something like virtio-net that is limited to the emulation of 1 device. For this reason, it is likely that the virtio-scmi backend would be a better fit in Xen itself, rather than run in userspace inside a VM.
Not sure what you mean when you say that SCMI requires a full view of the system.
SCMI can be used to read the status of resources in the system and typically leads to a full view of the system's resources.
If I assign the USB controller to a VM, I expect that VM to only "see" the USB controller and any attached USB peripherals, in addition to the other regular virtual resources that a VM commonly has.
If I assign SCMI to a VM, I expect the VM to "see" everything in the system thanks to the SCMI probing functions. Unless we only assign a single SCMI channel with limited capabilities to the VM, like EPAM's patch series on xen-devel is doing.
If you are referring to the system wide resources which reset or power up/down the whole SoC, this is not really what we are targeting here. Those system wide resources should already be handled by a dedicated power coprocessor. In our case, the IPs of the SoC will be handled by different VMs but those IPs are usually sharing common resources like a parent PLL , a power domain or a clock gating reg as few examples. Because all those VMs can't directly set these resources without taking into account others and because the power coprocessor doesn't have an unlimited number of channels, we add an SCMI backend that will gather and proxy the VM request before accessing the register that gates some clocks IP as an example or before powering down an external regulator shared between the camera and another device.
Do you know what would be the expected number of SCMI channels available in a "normal" deployment?
My expectation was that there would be enough SCMI channels to give one for each VM in a common embedded scenario, where the number of VMs is typically not very high. If we have enought channels so that we can assign each channel to a different VM maybe we can get away without a proxy?
This SCMI backend will most probably also send request with OSPM permission access to the power coprocessor once aggregating all the VMs ' request
Please correct me if I am wrong, but I would have expected the SCMI firmware to be able to do reference counting on the hardware resources and therefore be able to handle the case where:
- we have 2 VMs - each VM has its own SCMI channel - a VM requests power-off on 1 resource also used by the other VM
My understanding of the SCMI protocol is that the SCMI firmware implementation should detect that the resource in question is also in-use by another VM/channel and thus it would refuse the power-off operation. (For your information, that is also how the Xilinx EEMI protocol works.)
Reference counting is a key requirement for a good multi-channel implementation. If SCMI doesn't support it today, then we have a problem with SCMI multi-channel, regardless of virtualization.
We are using virtio-cmi protocol because it has the main advantage of not being tied to an hypervisor
That is a valuable goal, which is a bit different from the goal of finding the best SCMI architecture for Xen, and that's OK. Let's see if we can find any common ground and synergies we can exploit to improve both goals. I'll join the Stratos meeting tomorrow.
In our PoC, the SCMI backend is running with zehyr and reuse the same software that can run in the power coprocessor which helps splitting what is critical and must be handled by power coprocessor and what is not critical for the system (what is usually managed by linux directly when their no hypervisor involved typically)
FYI, a good and promising approach to handle both SCMI and SCPI is the series recently submitted by EPAM to mediate SCMI and SCPI requests in Xen: https://marc.info/?l=xen-devel&m=163947444032590
(Another "special" virtio backend is virtio-iommu for similar reasons: the guest p2m address mappings and also the IOMMU drivers are in Xen. It is not immediately clear whether a virtio-iommu backend would need to be in Xen or run as a process in dom0/domU.)
On the other hand, for all the other "normal" protocols (e.g. virtio-net, virtio-block, etc.) the backend would naturally run as a process in dom0 or domU (e.g. QEMU in Dom0) as one would expect.
Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
Unfortunately I have a regular conflict which prevents me from being able to join the Stratos calls. However, I can certainly make myself available for one call (unless something unexpected comes up).
- How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
I'll try to answer the question for a generic virtio frontend and backend instead (not SCMI because SCMI is unique due to the reasons above.)
Yes, xenstore is the easiest way to exchange configuration information between domains. I think EPAM used xenstore to exchange the configuration information in their virtio-block demo. There is a way to use xenstore even between dom0less VMs: https://marc.info/?l=xen-devel&m=164340547602391 Not just xenstore but full PV drivers too. However, in the dom0less case xenstore is going to become available some time after boot, not immediately at startup time. That's because you need to wait until xenstored is up and running.
There are other ways to send data from one VM to another which are available immediately at boot, such as Argo and static shared memory.
But dom0less is all about static partitioning, so it makes sense to exploit the build-time tools to the fullest. In the dom0less case, we already know what is going to run on the target before it is even turned on. As an example, we might have already prepared an environment with 3 VMs using Yocto and ImageBuilder. We could also generate all configurations needed and place them inside each VMs using Yocto's standard tools and ImageBuilder. So for dom0less, I recommend to go via a different route and pre-generate the configuration directly where needed instead of doing dynamic discovery.
- How to handle mapping of memory
AIUI the Xen model is the FE guest explicitly makes grant table requests to expose portions of it's memory to other domains. Can the BE query the hypervisor itself to discover the available grants or does it require coordination with Dom0/XenStore for that information to be available to the BE domain?
Typically the frontend passes grant table references to the backend (i.e. instead of plain guest physical addresses on the virtio ring.) Then, the backend maps the grants; Xen checks that the mapping is allowed.
We might be able to use the same model with virtio devices. A special pseudo-IOMMU driver in Linux would return a grant table reference and an offset as "DMA address". The "DMA address" is passed to the virtio backend over the virtio ring. The backend would map the grant table reference using the regular grant table hypercalls.
- How to handle signalling
I guess this requires a minimal implementation of the IOREQ calls for Zephyr so we can register the handler in the backend? Does the IOREQ API allow for a IPI style notifications using the global GIC IRQs?
Forgive the incomplete notes from the Stratos sync, I was trying to type while participating in the discussion so hopefully this email captures what was missed:
https://linaro.atlassian.net/wiki/spaces/STR/pages/28682518685/2022-02-03+Pr...
Yes, any emulation backend (including virtio backends) would require an IOREQ implementation, which includes notifications via event channels. Event channels are delivered as a GIC PPI interrupt to the Linux kernel. Then, the kernel sends the notification to userspace via a file descriptor.
On Wed, 16 Feb 2022 at 22:45, Stefano Stabellini stefano.stabellini@xilinx.com wrote:
On Tue, 15 Feb 2022, Vincent Guittot wrote:
On Tue, 8 Feb 2022 at 01:16, Stefano Stabellini stefano.stabellini@xilinx.com wrote:
On Mon, 7 Feb 2022, Alex Bennée wrote:
Hi Stefano,
Vincent gave an update on his virtio-scmi work at the last Stratos sync call and the discussion moved onto next steps.
Hi Alex,
I don't know the specifics of virtio-scmi, but if it is about power, clocks, reset, etc. like the original SCMI protocol, then virtio-scmi is
virtio-scmi is one transport channel that support SCMI protocol
likely going to be very different from all the other virtio frontends
The virtio-scmi front-end is merged mainline
and backends. That's because SCMI requires a full view of the system, which is different from something like virtio-net that is limited to the emulation of 1 device. For this reason, it is likely that the virtio-scmi backend would be a better fit in Xen itself, rather than run in userspace inside a VM.
Not sure what you mean when you say that SCMI requires a full view of the system.
SCMI can be used to read the status of resources in the system and typically leads to a full view of the system's resources.
If I assign the USB controller to a VM, I expect that VM to only "see" the USB controller and any attached USB peripherals, in addition to the other regular virtual resources that a VM commonly has.
If I assign SCMI to a VM, I expect the VM to "see" everything in the system thanks to the SCMI probing functions. Unless we only assign a single SCMI channel with limited capabilities to the VM, like EPAM's patch series on xen-devel is doing.
If you are referring to the system wide resources which reset or power up/down the whole SoC, this is not really what we are targeting here. Those system wide resources should already be handled by a dedicated power coprocessor. In our case, the IPs of the SoC will be handled by different VMs but those IPs are usually sharing common resources like a parent PLL , a power domain or a clock gating reg as few examples. Because all those VMs can't directly set these resources without taking into account others and because the power coprocessor doesn't have an unlimited number of channels, we add an SCMI backend that will gather and proxy the VM request before accessing the register that gates some clocks IP as an example or before powering down an external regulator shared between the camera and another device.
Do you know what would be the expected number of SCMI channels available in a "normal" deployment?
I don't have a fixed value but it can comes from few to hundreds depending of the product
My expectation was that there would be enough SCMI channels to give one for each VM in a common embedded scenario, where the number of VMs is typically not very high. If we have enought channels so that we can assign each channel to a different VM maybe we can get away without a proxy?
If you have enough HW channels per VM then you don't need anything including what EPAM is proposing. But part of my requirement is that we don't have enough HW resources and we don't want to update the secure power coprocessor for each and every product.
In SCMI, each channel/agent can have its own view of available resources; Typically only the ATF/PSCI channel can power off the whole system but not OSPMs agent
This SCMI backend will most probably also send request with OSPM permission access to the power coprocessor once aggregating all the VMs ' request
Please correct me if I am wrong, but I would have expected the SCMI firmware to be able to do reference counting on the hardware resources and therefore be able to handle the case where:
The example that I have in mind is : current case: Linux takes care of a clock divider that provides clocks for let say video decoder and SD controller but its parent is a system clock shared with others subsystem which is managed by the power coprocessor. Linux will 1st send a request to the coprocessor to enable system clock. Then it will set the divider and probably ungates the clock at HW IP level
Now we move the sdcard in VMA and the video decoder in VMB. The SCMI server VM will take care of the clock divider and clock gating register VMA will send a request to SCMI backend which aggregates/refcounts its request with VMB request. When it needs to enable the clock divider, It will 1st send a request to the coprocessor for the system clock.
The coprocessor stays unchanged and the SCMI backend can be adjusted per product.
- we have 2 VMs
- each VM has its own SCMI channel
- a VM requests power-off on 1 resource also used by the other VM
yes it has refcounting but you assume that you have enough HW channels. It also implies that the coprocessor firmware is different for each and every end product because you will have to adjust the scmi permission of each channel
My understanding of the SCMI protocol is that the SCMI firmware implementation should detect that the resource in question is also in-use by another VM/channel and thus it would refuse the power-off operation. (For your information, that is also how the Xilinx EEMI protocol works.)
Reference counting is a key requirement for a good multi-channel implementation. If SCMI doesn't support it today, then we have a problem with SCMI multi-channel, regardless of virtualization.channels
We are using virtio-cmi protocol because it has the main advantage of not being tied to an hypervisor
That is a valuable goal, which is a bit different from the goal of finding the best SCMI architecture for Xen, and that's OK. Let's see if we can find any common ground and synergies we can exploit to improve both goals. I'll join the Stratos meeting tomorrow.
Great
In our PoC, the SCMI backend is running with zehyr and reuse the same software that can run in the power coprocessor which helps splitting what is critical and must be handled by power coprocessor and what is not critical for the system (what is usually managed by linux directly when their no hypervisor involved typically)
FYI, a good and promising approach to handle both SCMI and SCPI is the series recently submitted by EPAM to mediate SCMI and SCPI requests in Xen: https://marc.info/?l=xen-devel&m=163947444032590
(Another "special" virtio backend is virtio-iommu for similar reasons: the guest p2m address mappings and also the IOMMU drivers are in Xen. It is not immediately clear whether a virtio-iommu backend would need to be in Xen or run as a process in dom0/domU.)
On the other hand, for all the other "normal" protocols (e.g. virtio-net, virtio-block, etc.) the backend would naturally run as a process in dom0 or domU (e.g. QEMU in Dom0) as one would expect.
Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
Unfortunately I have a regular conflict which prevents me from being able to join the Stratos calls. However, I can certainly make myself available for one call (unless something unexpected comes up).
- How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
I'll try to answer the question for a generic virtio frontend and backend instead (not SCMI because SCMI is unique due to the reasons above.)
Yes, xenstore is the easiest way to exchange configuration information between domains. I think EPAM used xenstore to exchange the configuration information in their virtio-block demo. There is a way to use xenstore even between dom0less VMs: https://marc.info/?l=xen-devel&m=164340547602391 Not just xenstore but full PV drivers too. However, in the dom0less case xenstore is going to become available some time after boot, not immediately at startup time. That's because you need to wait until xenstored is up and running.
There are other ways to send data from one VM to another which are available immediately at boot, such as Argo and static shared memory.
But dom0less is all about static partitioning, so it makes sense to exploit the build-time tools to the fullest. In the dom0less case, we already know what is going to run on the target before it is even turned on. As an example, we might have already prepared an environment with 3 VMs using Yocto and ImageBuilder. We could also generate all configurations needed and place them inside each VMs using Yocto's standard tools and ImageBuilder. So for dom0less, I recommend to go via a different route and pre-generate the configuration directly where needed instead of doing dynamic discovery.
- How to handle mapping of memory
AIUI the Xen model is the FE guest explicitly makes grant table requests to expose portions of it's memory to other domains. Can the BE query the hypervisor itself to discover the available grants or does it require coordination with Dom0/XenStore for that information to be available to the BE domain?
Typically the frontend passes grant table references to the backend (i.e. instead of plain guest physical addresses on the virtio ring.) Then, the backend maps the grants; Xen checks that the mapping is allowed.
We might be able to use the same model with virtio devices. A special pseudo-IOMMU driver in Linux would return a grant table reference and an offset as "DMA address". The "DMA address" is passed to the virtio backend over the virtio ring. The backend would map the grant table reference using the regular grant table hypercalls.
- How to handle signalling
I guess this requires a minimal implementation of the IOREQ calls for Zephyr so we can register the handler in the backend? Does the IOREQ API allow for a IPI style notifications using the global GIC IRQs?
Forgive the incomplete notes from the Stratos sync, I was trying to type while participating in the discussion so hopefully this email captures what was missed:
https://linaro.atlassian.net/wiki/spaces/STR/pages/28682518685/2022-02-03+Pr...
Yes, any emulation backend (including virtio backends) would require an IOREQ implementation, which includes notifications via event channels. Event channels are delivered as a GIC PPI interrupt to the Linux kernel. Then, the kernel sends the notification to userspace via a file descriptor.
Hi Vincent,
I am replying to this thread to follow-up on this morning's discussion.
I am attaching the simple patch that I mentioned during the call to add event channels support to guest kernels, see xen.h.
I am also attaching a toy example application that makes use of it, just to give you an idea.
Cheers,
Stefano
Hi All,
Sorry for the late reply but I was off last week. I will go through the thread and try to answer open point
On Mon, 7 Feb 2022 at 11:56, Alex Bennée alex.bennee@linaro.org wrote:
Hi Stefano,
Vincent gave an update on his virtio-scmi work at the last Stratos sync call and the discussion moved onto next steps. Currently the demo setup is intermediated by a double-ended vhost-user daemon running on the devbox acting as a go between a number of QEMU instances representing the front and back-ends. You can view the architecture with Vincents diagram here:
https://docs.google.com/drawings/d/1YSuJUSjEdTi2oEUq4oG4A9pBKSEJTAp6hhcHKKhm...
The key virtq handling is done over the special carve outs of shared memory between the front end and guest. However the signalling is currently over a virtio device on the backend. This is useful for the PoC but obviously in a real system we don't have a hidden POSIX system acting as a go between not to mention the additional latency it causes with all those context switches.
I was hoping we could get some more of the Xen experts to the next Stratos sync (17th Feb) to go over approaches for a properly hosted on Xen approach. From my recollection (Vincent please correct me if I'm wrong) of last week the issues that need solving are:
- How to handle configuration steps as FE guests come up
The SCMI server will be a long running persistent backend because it is managing real HW resources. However the guests may be ephemeral (or just restarted) so we can't just hard-code everything in a DTB. While the virtio-negotiation in the config space covers most things we still need information like where in the guests address space the shared memory lives and at what offset into that the queues are created. As far as I'm aware the canonical source of domain information is XenStore (https://wiki.xenproject.org/wiki/XenStore) but this relies on a Dom0 type approach. Is there an alternative for dom0less systems or do we need a dom0-light approach, for example using STR-21 (Ensure Zephyr can run cleanly as a Dom0 guest) providing just enough services for FE's to register metadata and BE's to read it?
- How to handle mapping of memory
AIUI the Xen model is the FE guest explicitly makes grant table requests to expose portions of it's memory to other domains. Can the BE query the hypervisor itself to discover the available grants or does it require coordination with Dom0/XenStore for that information to be available to the BE domain?
I have noticed that it was possible to share memory between VMs in the VM config file which seem to be quite similar to what is done with qemu to share memory object between VMs
- How to handle signalling
I guess this requires a minimal implementation of the IOREQ calls for Zephyr so we can register the handler in the backend? Does the IOREQ API allow for a IPI style notifications using the global GIC IRQs?
Forgive the incomplete notes from the Stratos sync, I was trying to type while participating in the discussion so hopefully this email captures what was missed:
https://linaro.atlassian.net/wiki/spaces/STR/pages/28682518685/2022-02-03+Pr...
Vincent, anything to add?
I want to use an interface that is not tied to an hypervisor that's why i have reused the virtio_mmio to emulate the device side where the backend can get virtqueue description
-- Alex Bennée
stratos-dev@op-lists.linaro.org
-
Alex Bennée -
Stefano Stabellini -
Stefano Stabellini -
Vincent Guittot