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Re: [PATCH v1 3/4] arm/sysctl: Implement cpu hotplug ops
- To: Julien Grall <julien@xxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 25 Sep 2025 07:44:21 +0200
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 25 Sep 2025 05:44:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 23.09.2025 20:41, Julien Grall wrote:
> On 18/09/2025 15:51, Jan Beulich wrote:
>> On 18.09.2025 15:35, Julien Grall wrote:
>>> On 18/09/2025 13:16, Mykyta Poturai wrote:
>>>> +static long cpu_hotplug_sysctl(struct xen_sysctl_cpu_hotplug *hotplug)
>>>> +{
>>>> + bool up;
>>>> +
>>>> + switch (hotplug->op) {
>>>> + case XEN_SYSCTL_CPU_HOTPLUG_ONLINE:
>>>> + if ( hotplug->cpu == 0 )
>>>
>>> I can't find a similar check on x86. Do you have any pointer?
>>
>> When CPU 0 cannot be brought down (see cpu_down()), tryin to bring it up
>> is kind of pointless, and hence can perhaps be short circuited like this?
>
> Thanks for the clarification, I missed the check in cpu_down(). That
> said, I don't see any value to short circuit it. In fact, I see this as
> more a risk because if we ever decide to allow CPU 0 to be offlined,
> then it would be more difficult to find places where we short circuit it.
>
> So I would rather prefer if we remove the checks.
In fact I agree (and I merely wanted to point out the present situation):
CPU0 not (normally) being possible to be brought down is, I think, pretty
much an x86 thing. I.e. I think the check would want to go away from
common code.
Jan
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