Re: [PATCH v3 0/5] Implement CPU hotplug on Arm

[Mykola Kvach <xakep.amatop@xxxxxxxxx>]

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.

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.

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.

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.



On Fri, Oct 10, 2025 at 12:36 PM Mykyta Poturai <Mykyta_Poturai@xxxxxxxx> wrote:
>
> This series implements support for CPU hotplug/unplug on Arm. To achieve this,
> several things need to be done:
>
> 1. XEN_SYSCTL_CPU_HOTPLUG_* calls implemented.
> 2. timer and GIC maintenance interrupts switched to static irqactions to 
> remove
> the need for freeing them during release_irq.
> 3. Enabled the build of xen-hptool on Arm.
>
> Tested on QEMU.
>
> v2->v3:
> * add docs
>
> v1->v2:
> * see individual patches
>
> Mykyta Poturai (5):
>   arm/time: Use static irqaction
>   arm/gic: Use static irqaction
>   arm/sysctl: Implement cpu hotplug ops
>   tools: Allow building xen-hptool without CONFIG_MIGRATE
>   docs: Document CPU hotplug
>
>  config/Tools.mk.in               |  1 +
>  docs/misc/cpu-hotplug.txt        | 51 ++++++++++++++++++++++++++++++++
>  tools/configure                  | 30 +++++++++++++++++++
>  tools/configure.ac               |  1 +
>  tools/libs/guest/Makefile.common |  4 +++
>  tools/misc/Makefile              |  2 +-
>  xen/arch/arm/gic.c               | 11 +++++--
>  xen/arch/arm/sysctl.c            | 45 ++++++++++++++++++++++++++++
>  xen/arch/arm/time.c              | 21 ++++++++++---
>  9 files changed, 159 insertions(+), 7 deletions(-)
>  create mode 100644 docs/misc/cpu-hotplug.txt
>  mode change 100755 => 100644 tools/configure
>
> --
> 2.34.1
>

Best regards,
Mykola