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Re: [RFC PATCH 2/7] common: Track latest pCPU that ran the vCPU
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 5 May 2026 12:30:03 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Jason Andryuk <jason.andryuk@xxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 05 May 2026 10:30:12 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 05.05.2026 12:17, Teddy Astie wrote:
> Le 04/05/2026 à 17:51, Jan Beulich a écrit :
>> On 15.04.2026 15:32, Teddy Astie wrote:
>>> Track on which pCPU each vCPU of a domain ran. This will
>>> be used to know whether a TLB flush is required or not
>>> when the vCPU is migrated on another pCPU.
>>
>> Somewhat related tracking already exists - see the dirty_cpumask field.
>
> I've seen it, but I'm not sure how it can be leveraged here.
>
> I will try to take a closer look if that could be used instead.
>
>> But what title and description say doesn't match ...
>>
>>> @@ -977,6 +978,8 @@ void asmlinkage svm_vmenter_helper(void)
>>>
>>> svm_sync_vmcb(curr, vmcb_needs_vmsave);
>>>
>>> + curr->domain->latest_vcpu[cpu] = curr->vcpu_id;
>>
>> ... the implementation anyway: You track which vCPU last ran on a given
>> pCPU. Since the same pCPU may have run multiple vCPU-s which then weren't
>> scheduled again, you lose data afaict.
>>
>
> I mixed up the wording. But the implementation is the proper intent.
>
> It's more
>
> Track which vCPU of the domain each pCPU ran.
Okay, yet then (as already pointed out) how do you know vCPU0 ran last on
a given pCPU if after its de-scheduling vCPU1 (of the same domain) was
put there. Your track record (after de-scheduling vCPU1) will say only
vCPU1; information on vCPU0 will be lost. Yet then, as also indicated,
it's not quite clear to me how exactly you mean to leverage this tracking.
Jan
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