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Re: [PATCH v3 0/5] Implement CPU hotplug on Arm
- To: Mykola Kvach <xakep.amatop@xxxxxxxxx>
- From: Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>
- Date: Mon, 20 Oct 2025 14:15:23 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, "Daniel P. Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 20 Oct 2025 14:15:43 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHcOcdPI3ki3mxsgkWwwomwd2VOcrTDfxsAgAelSIA=
- Thread-topic: [PATCH v3 0/5] Implement CPU hotplug on Arm
On 15.10.25 20:30, Mykola Kvach wrote:
> Hi Mykyta,
>
> Thanks for the series.
>
> It seems there might be issues here -- please take a look and let me
> know if my concerns are valid:
>
> 1. FF-A notification IRQ: after a CPU down->up cycle the IRQ
> configuration may be lost.
OPTEE and FFA are marked as unsupported.
> 2. GICv3 LPIs: a CPU may fail to come back up unless its LPI pending
> table exists (is allocated) on bring-up. See
> gicv3_lpi_allocate_pendtable() and its call chain.
ITS is marked as unsupported. I have a plan to deal with this, but it is
out of scope of this series.
> 3. IRQ migration on CPU down: if an IRQ targets a CPU being offlined,
> its affinity should be moved to an online CPU before completing the
> offlining.
All guest tied IRQ migration is handled by the scheduler. Regarding the
irqs used by Xen, I didn't find any with affinity to other CPUs than CPU
0, which can't be disabled. I think theoretically it is possible for
them to have different affinity, but it seems unlikely considering that
x86 hotplug code also doesn't seem to do any Xen irq migration AFAIU.
> 4. Race between the new hypercalls and disable/enable_nonboot_cpus():
> disable_nonboot_cpus is called, enable_nonboot_cpus() reads
> frozen_cpus, and before it calls cpu_up() a hypercall onlines the CPU.
> cpu_up() then fails as "already online", but the CPU_RESUME_FAILED
> path may still run for an already-online CPU, risking use-after-free
> of per-CPU state (e.g. via free_percpu_area()) and other issues
> related to CPU_RESUME_FAILED notification.
>
There don't seem to be any calls to disable/enable_nonboot_cpus() on
Arm. If we take x86 as an example, then they are called with all domains
already paused, and I don't see how paused domains can issue hypercalls.
>
> Best regards,
> Mykola
--
Mykyta
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