Re: [PATCH v5 0/5] Implement CPU hotplug on Arm

[Bertrand Marquis <Bertrand.Marquis@arm.com>]

Hi Mykyta,

> On 13 Jan 2026, at 09:45, Mykyta Poturai <Mykyta_Poturai@epam.com> 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 on Arm64.
> 2. Enabled building of xen-hptool.
> 3. Migration of irqs from dying CPUs implemented.
> 
> Tested on QEMU.

Do you have a way to validate this on real hardware ?
Doing hotplug/unplug might have side effects (caches, coherency, interrupts, etc) that
I would not be completely confident to have validated by only testing this in qemu.

Could you explain how you validated this (what kind of tests you did).

I would be interested to know how the system behave in the following cases:
- stopping cpu with guests pinned to specific cores
- stopping a cpu with a guest pinned and only running on that one
- checking with passthrough devices (for irq balancing)

Then a general remark i have is that most of this code (if not all) is always in Xen
even if CONFIG_CPU_HOTPLUG is not activated in xen config which will probably
generate some dead code. Could we protect some of the code not used when
HOTPLUG is not activated with if (is_enabled(CONFIG_CPU_HOTPLUG)) or
equivalent forms ?

Cheers
Bertrand

> 
> v4->v5:
> * drop merged patches
> * combine "smp: Move cpu_up/down helpers to common code" with 
>  "arm/sysctl: Implement cpu hotplug ops"
> * see individual patches
> 
> v3->v4:
> * add irq migration patches
> * see individual patches
> 
> v2->v3:
> * add docs
> 
> v1->v2:
> * see individual patches
> 
> Mykyta Poturai (5):
>  arm/irq: Keep track of irq affinities
>  arm/irq: Migrate IRQs during CPU up/down operations
>  arm/sysctl: Implement cpu hotplug ops
>  tools: Allow building xen-hptool without CONFIG_MIGRATE
>  docs: Document CPU hotplug
> 
> SUPPORT.md                       |  1 +
> docs/misc/cpu-hotplug.txt        | 50 +++++++++++++++++++++++++
> tools/libs/guest/Makefile.common |  2 +-
> tools/misc/Makefile              |  2 +-
> xen/arch/arm/Kconfig             |  1 +
> xen/arch/arm/gic-vgic.c          |  2 +
> xen/arch/arm/include/asm/irq.h   |  2 +
> xen/arch/arm/irq.c               | 63 +++++++++++++++++++++++++++++++-
> xen/arch/arm/smp.c               |  9 +++++
> xen/arch/arm/smpboot.c           |  6 +++
> xen/arch/arm/vgic.c              | 14 ++++++-
> xen/arch/ppc/stubs.c             |  4 ++
> xen/arch/riscv/stubs.c           |  5 +++
> xen/arch/x86/Kconfig             |  1 +
> xen/arch/x86/include/asm/smp.h   |  3 --
> xen/arch/x86/smp.c               | 33 ++---------------
> xen/arch/x86/sysctl.c            | 12 ++----
> xen/common/Kconfig               |  3 ++
> xen/common/smp.c                 | 34 +++++++++++++++++
> xen/common/sysctl.c              | 45 +++++++++++++++++++++++
> xen/include/xen/smp.h            |  4 ++
> 21 files changed, 248 insertions(+), 48 deletions(-)
> create mode 100644 docs/misc/cpu-hotplug.txt
> 
> -- 
> 2.51.2