Hi James,
From: James Morse [mailto:james.morse@arm.com] Sent: Friday, October 28, 2022 5:17 PM To: Salil Mehta salil.mehta@huawei.com; joyce.qi@linaro.org Cc: linaro-open-discussions@op-lists.linaro.org; lorenzo.pieralisi@linaro.org; ilkka@os.amperecomputing.com; Jean-Philippe Brucker jean-philippe.brucker@arm.com; salil.mehta@opnsrc.net Subject: Re: [Linaro-open-discussions] Re: Invitation: Linaro Open Discussions monthly meeting @ Wed 5 Oct 2022 19:00 - 20:00 (HKT) (linaro-open-discussions@op-lists.linaro.org)
Hi Salil,
Thanks for this, I've fixed a couple of bugs in the tree.
On the qemu side: Using KVM and your Qemu tree, PSCI returns success, when it should return denied for CPUs that are forbidden due to firmware policy: [ 0.027597] smp: Bringing up secondary CPUs ... [ 5.108204] CPU1: failed to come online [ 5.109198] CPU1: failed in unknown state : 0x0 [ 5.110384] smp: Brought up 1 node, 1 CPU
Did you try to use below branch(with some suggested fixes) which I shared with you?
[1] James Approach with online-capable and present==possible (some fixes) https://github.com/salil-mehta/linux.git virt-cpuhp-arm64/rfc-v2/jmorse-pres-eq-poss-cpu
This is mostly harmless for linux, but using PSCI_DENIED squashes the error, and avoids the timeout waiting for the secondary to turn up. Mostly harmless as this may cause 'cpus stuck in kernel' to get set, which would prevent kexec.
This probably means you're not using the HVC_TO_USER support for KVM that gets added as part of this series. You can then use the PSCI_TO_USER cap to disable the kernel's PSCI handler, which lets the VMM manage this directly.
Yes, that is true . I intentionally removed the PSCI-to-userspace patches as I was wondering if the CPU devices are not available in the guest kernel for the disabled CPUs how can they be made online?
Still using KVM, firmware should clear the enabled bit from _STA for any call after the eject-request. I see this is still 0xF before _EJx is called.
This is how the existing handshake protocol works. Firmware intimates the OS about ejection request which confirms its status about ejection-in-progress. Firmware/VMM then sends the ACPI event to remove the CPU from the kernel(which effectively is try-to-offline process and making not-present), Until this point _STA.ENABLED=1 (which is a correct behavior since firmware/VMM cannot disable the CPU before it has been made offline by the OS - can we snatch the physical resources before completing the housekeeping at OS?). Once offline'ing/removal is done, OS then informs the firmware/VMM about this completion using _EJ0 method (Which is effective way of giving green signal to the firmware/VMM to go ahead and remove the physical resources).
Firmware then removes the device and any further evaluation of _STA would show _STA.ENABLED=0 and ideally should show is not present(but this is something which I have not changed in the QEMU - maybe we need to do or does it even matter at QEMU level?).
This means the cpu remains registered, and user-space doesn't get notified that the CPU is no longer available. User-space can still try to enable the CPU, which will fail.
That is true and to fix this we could use the online-capable Bit instead of using the _STA.ENABLED Bit status as discussed in the 5th Oct LOD meeting. I have done those changes in the above branch [1].
Thanks Salil
I've not tried TCG mode yet.
Thanks,
James
On 17/10/2022 17:12, Salil Mehta via Linaro-open-discussions wrote:
Hi James, As discussed in the meeting below are the repositories which you might want
to have a look.
[1] Forward ported QEMU with some fixes was shared (by Salil) https://github.com/salil-mehta/qemu.git
virt-cpuhp-armv8/rfc-v1-port29092022
[2] James Approach with online-capable and present==possible (some fixes) https://github.com/salil-mehta/linux.git
virt-cpuhp-arm64/rfc-v2/jmorse-pres-eq-poss-cpu
[3] Variant of James approach with online-capable and conditionally present
cpus
https://github.com/salil-mehta/linux.gitvirt-cpuhp-arm64/rfc-v2/jmorse-variant-with-cond-present-cpu
I have also shared the LOD presentation with Joyce for uploading to the site.
Attachment: [1] Linaro Open Discussion Meeting Update - 05102022 - Salil_Mehta-fixed.pdf
Many thanks Salil
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