Re: [PATCH v1 0/2] domctl: Avoid taking domctl lock for certain ops used during migration

[Jan Beulich <jbeulich@suse.com>]

On 15.06.2026 13:17, Ross Lagerwall wrote:
> On 6/11/26 5:06 PM, Jan Beulich wrote:
>> On 11.06.2026 18:02, Ross Lagerwall wrote:
>>> On 6/11/26 3:55 PM, Jan Beulich wrote:
>>>> On 09.06.2026 17:15, Ross Lagerwall wrote:
>>>>> When performing multiple migrations in parallel, the domctl lock may
>>>>> become extremely contended:
>>>>>
>>>>> * Operations like "xl vcpu-list" were observed to take in excess of 20s
>>>>>     to execute.
>>>>
>>>> Does "xl vcpu-list" involve ...
>>>>
>>>>> * The "clean" shadow op may pause the domain, restart with a
>>>>>     continuation and then become blocked on the domctl lock, causing VM
>>>>>     downtime in excess of 20 seconds.
>>>>>
>>>>> These issues can be fixed by not holding the domctl for the frequently
>>>>> called operations during migration.
>>>>>
>>>>> Thanks
>>>>>
>>>>> Ross Lagerwall (2):
>>>>>     domctl: Handle XEN_DOMCTL_getpageframeinfo3 without the domctl lock
>>>>
>>>> ... XEN_DOMCTL_getpageframeinfo3?
>>>>
>>>
>>> No, but "xl vcpu-list" takes the domctl lock
>>
>> If this is still the case after XSA-492, then maybe the follow-ups I have
>> pending to post will eliminate (or at least reduce) this. I don't think
>> that's 4.22 material, though.
> 
> Yes, AFAICT "xl vcpu-list" calls XEN_DOMCTL_getvcpuinfo and
> XEN_DOMCTL_getvcpuaffinity for each domain and both of these still take
> the domctl lock.

XEN_DOMCTL_getvcpuinfo is taken care of by my to-be-posted follow-on.
XEN_DOMCTL_[gs]etvcpuaffinity first need to have their own locking added,
before their handling can be moved out.

Jan