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Re: [PATCH v5 0/5] Implement CPU hotplug on Arm
- To: Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>
- From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
- Date: Wed, 4 Feb 2026 13:02:36 +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 13:04:02 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH v5 0/5] Implement CPU hotplug on Arm
Hi Mykyta,
> On 4 Feb 2026, at 13:58, Mykyta Poturai <Mykyta_Poturai@xxxxxxxx> wrote:
>
> 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.
Thanks.
>
> 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.
If a guest has an hardware device assigned using passthrough and one of
its core is removed, is the guest and hardware device continue to work
well after ?
Cheers
Bertrand
>
>
> --
> Mykyta
|
