On Mon, Mar 06, 2023 at 04:40:24PM +0530, Viresh Kumar wrote:
The current model of memory mapping at the back-end works fine where a standard call to mmap() (for the respective file descriptor) is enough before the front-end can start accessing the guest memory.
There are other complex cases though where the back-end needs more information and simple mmap() isn't enough. For example Xen, a type-1 hypervisor, currently supports memory mapping via two different methods, foreign-mapping (via /dev/privcmd) and grant-dev (via /dev/gntdev). In both these cases, the back-end needs to call mmap() and ioctl(), and need to pass extra information via the ioctl(), like the Xen domain-id of the guest whose memory we are trying to map.
Add a new protocol feature, 'VHOST_USER_PROTOCOL_F_XEN_MMAP', which lets the back-end know about the additional memory mapping requirements. When this feature is negotiated, the front-end can send the 'VHOST_USER_SET_XEN_MMAP' message type to provide the additional information to the back-end.
Signed-off-by: Viresh Kumar viresh.kumar@linaro.org
V1->V2:
- Make the custom mmap feature Xen specific, instead of being generic.
- Clearly define which memory regions are impacted by this change.
- Allow VHOST_USER_SET_XEN_MMAP to be called multiple times.
- Additional Bit(2) property in flags.
docs/interop/vhost-user.rst | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+)
diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst index 3f18ab424eb0..8be5f5eae941 100644 --- a/docs/interop/vhost-user.rst +++ b/docs/interop/vhost-user.rst @@ -258,6 +258,24 @@ Inflight description :queue size: a 16-bit size of virtqueues +Xen mmap description +^^^^^^^^^^^^^^^^^^^^
++-------+-------+ +| flags | domid | ++-------+-------+
+:flags: 64-bit bit field
+- Bit 0 is set for Xen foreign memory memory mapping. +- Bit 1 is set for Xen grant memory memory mapping. +- Bit 2 is set if the back-end can directly map additional memory (like
- descriptor buffers or indirect descriptors, which aren't part of already
- shared memory regions) without the need of front-end sending an additional
- memory region first.
I don't understand what Bit 2 does. Can you rephrase this? It's unclear to me how additional memory can be mapped without a memory region (especially the fd) is sent?
+:domid: a 64-bit Xen hypervisor specific domain id.
C structure
@@ -867,6 +885,7 @@ Protocol features #define VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS 14 #define VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS 15 #define VHOST_USER_PROTOCOL_F_STATUS 16
- #define VHOST_USER_PROTOCOL_F_XEN_MMAP 17
Front-end message types
@@ -1422,6 +1441,23 @@ Front-end message types query the back-end for its device status as defined in the Virtio specification. +``VHOST_USER_SET_XEN_MMAP``
- :id: 41
- :equivalent ioctl: N/A
- :request payload: Xen mmap description
- :reply payload: N/A
- When the ``VHOST_USER_PROTOCOL_F_XEN_MMAP`` protocol feature has been
- successfully negotiated, this message is submitted by the front-end to set the
- Xen hypervisor specific memory mapping configurations at the back-end. These
- configurations should be used to mmap memory regions, virtqueues, descriptors
- and descriptor buffers. The front-end must send this message before any
- memory-regions are sent to the back-end via ``VHOST_USER_SET_MEM_TABLE`` or
- ``VHOST_USER_ADD_MEM_REG`` message types. The front-end can send this message
- multiple times, if different mmap configurations are required for different
- memory regions, where the most recent ``VHOST_USER_SET_XEN_MMAP`` must be used
- by the back-end to map any newly shared memory regions.
This message modifies the behavior of subsequent VHOST_USER_SET_MEM_TABLE and VHOST_USER_ADD_MEM_REG messages. The memory region structs can be extended and then VHOST_USER_SET_XEN_MMAP isn't needed.
In other words:
When VHOST_USER_PROTOCOL_F_XEN_MMAP is negotiated, each "Memory regions description" and "Single memory region description" has the following additional fields appended:
+----------------+-------+ | xen_mmap_flags | domid | +----------------+-------+
:xen_mmap_flags: 64-bit bit field :domid: a 64-bit Xen hypervisor specific domain id.
Stefan