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Re: [PATCH v5 0/5] Implement CPU hotplug on Arm
- To: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
- From: Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>
- Date: Wed, 4 Feb 2026 12:58:46 +0000
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Timothy Pearson <tpearson@xxxxxxxxxxxxxxxxxxxxx>, Alistair Francis <alistair.francis@xxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, "Daniel P. Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>
- Delivery-date: Wed, 04 Feb 2026 12:58:58 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHchGj6Bf8iLxj9KkKeGyrpjQwglLVxTT0AgAFV7wA=
- Thread-topic: [PATCH v5 0/5] Implement CPU hotplug on Arm
On 03.02.26 18:35, Bertrand Marquis wrote:
> Hi Mykyta,
>
>> On 13 Jan 2026, at 09:45, 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 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
>
Hi Bertrand
I’ve tested various pinning scenarios, and all of them work fine, as Xen
forcibly unpins everything that was pinned to the offlined CPU.
I’ve also explicitly checked that interrupts are getting moved to and
from cores when needed.
I will update the next cover letter to explain testing better and also
try to test on some hardware to be sure.
Can you please elaborate a little on what exactly you mean by “checking
with passthrough devices”? I suspect I am not fully getting the idea.
--
Mykyta
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