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Re: [PATCH 0/3] Change XEN_DOMCTL_setvcpuaffinity interface
- To: Jürgen Groß <jgross@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 1 Jul 2026 15:48:35 +0200
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- Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, George Dunlap <gwd@xxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 01 Jul 2026 13:48:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 01.07.2026 15:08, Jürgen Groß wrote:
> On 01.07.26 13:39, Jan Beulich wrote:
>> On 01.07.2026 13:16, Juergen Gross wrote:
>>> Today the XEN_DOMCTL_setvcpuaffinity hypercall will not only change the
>>> affinity settings of a vcpu, it will return the new effective settings,
>>> too.
>>>
>>> In case an error happens during this hypercall, the expectation of the
>>> caller is that the affinity of the vcpu didn't change. This isn't true,
>>> however, if passing the new effective affinity back to the user is
>>> failing.
>>
>> That'll be the caller's fault though. Any -EFAULT coming back are an
>> indication that the caller needs fixing.
>>
>>> Instead of making error handling in the hypervisor even more complex,
>>> just change the interface by NOT passing back the new affinity. It can
>>> easily be obtained by another XEN_DOMCTL_getvcpuaffinity call if
>>> needed.
>>
>> At the expense of further increasing the pressure on the domctl lock.
>> When making the recent error code change in this area, I was actually
>> wondering in how far these two sub-ops might also be candidates for
>> moving out of the locked region. Yet with vcpu_affinity_domctl() not
>> using any locking that didn't look entirely straightforward to answer.
>
> The calling hierarchy for the XEN_DOMCTL_setvcpuaffinity case is:
>
> vcpu_affinity_domctl()
> vcpu_set_hard_affinity()
> vcpu_set_affinity()
> vcpu_set_affinity()
>
> vcpu_set_affinity() does the correct locking (takes the unit lock).
But that's not sufficient: The two involved vcpu_set_affinity() calls
want to stay together, i.e. not race with another domctl.
Jan
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