Re: [PATCH RFC] KVM: arm64: PMU: Use multiple host PMUs

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2025/03/20 3:51, Oliver Upton wrote:
> On Wed, Mar 19, 2025 at 06:38:38PM +0000, Marc Zyngier wrote:
>> On Wed, 19 Mar 2025 11:51:21 +0000, Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>> What about setting the flag automatically when a user fails to pin
>>> vCPUs to CPUs that are covered by one PMU? There would be no change if
>>> a user correctly pins vCPUs as it is. Otherwise, they will see a
>>> correct feature set advertised to the guest and the cycle counter
>>> working.
>>
>> How do you know that the affinity is "correct"? VCPU affinity can be
>> changed at any time. I, for one, do not want my VMs to change
>> behaviour because I let the vcpus bounce around as the scheduler sees
>> fit.

Checking the affinity when picking the default PMU; the vCPU affinity is 
the only thing that rules the choice of the default PMU even now.

Perhaps we may model the API as follows: introduce another "composite" 
PMU that works on any core but only exposes the cycle counter. Robust 
VMMs will choose it or one of hardware PMUs with 
KVM_ARM_VCPU_PMU_V3_SET_PMU. KVM will choose the default PMU according 
to the vCPU affinity at the point of KVM_ARM_VCPU_INIT otherwise. If the 
affinity is covered by one hardware PMU, that PMU will be chosen as the 
default. The "composite" PMU will be the default otherwise.

Regards,
Akihiko Odaki

>>
>> Honestly, this is not a can of worm I want to open. We already have a
>> pretty terrible userspace API for the PMU, let's not add to the
>> confusion. *If* we are going down the road of presenting a dumbed-down
>> PMU to the guest, it has to be an explicit buy-in from userspace.
> 
> I also have a very strong distaste for the crappy UAPI we have around a
> 'default' PMU. At the same time I do recognize this hurts practical
> usecases since some VMMs can't be bothered to configure the vPMU / vCPU
> pinning correctly.
> 
> I'm at least willing to plug my nose and do the following:
> 
>   1) When the VMM does not specify a vPMU type:
> 
>     - We continue to present the 'default' PMU (including event counters)
>       to the VM
> 
>     - KVM ensures that the fixed CPU cycle counter works on any PMUv3
>       implementation in the system, even if it is different from the
>       default
> 
>     - Otherwise, event counters will only count on the default
>       implementation and will not count on different PMUs
> 
>   2) Implement your suggestion of a UAPI where the VMM can select a PMU
>      that only has the CPU cycle counter and works on any PMUv3
>      implementation.
> 
> Either way KVM will need to have some special case handling of the fixed
> CPU cycle counter. That'd allow users to actually run Windows *now* and
> provide a clear mechanism for userspace to present a less-broken vPMU if
> it cares.
> 
> Thanks,
> Oliver