Re: [PATCH 1/2] KVM: x86/pmu: Reset perf_capabilities in vcpu to 0 if PDCM is disabled

Paolo Bonzini
2024-01-29T14:39:13Z

On 1/24/24 23:51, Sean Christopherson wrote:
>> If we follow the suggestion by removing the initial value at vCPU
>> creation time, then I think it breaks the existing VMM code, since that
>> requires VMM to explicitly set the MSR, which I am not sure we do today.
> Yeah, I'm hoping we can squeak by without breaking existing setups.
>
> I'm 99% certain QEMU is ok, as QEMU has explicitly set MSR_IA32_PERF_CAPABILITIES
> since support for PDCM/PERF_CAPABILITIES was added by commit ea39f9b643
> ("target/i386: define a new MSR based feature word - FEAT_PERF_CAPABILITIES").
>
> Frankly, if our VMM doesn't do the same, then it's wildly busted. Relying on
> KVM to define the vCPU is irresponsible, to put it nicely.

Yes, I tend to agree.

What QEMU does goes from the squeaky clean to the very debatable
depending on the parameters you give it.

With "-cpu Haswell" and similar, it will provide values for all CPUID
and MSR bits that match as much as possible values from an actual CPU
model. It will complain if there are some values that do not match[1].

With "-cpu host", it will copy values from KVM_GET_SUPPORTED_CPUID and
from the feature MSRs, but only for features that it knows about.

With "-cpu host,migratable=no", it will copy values from
KVM_GET_SUPPORTED_CPUID and from the feature MSRs, but only for *feature
words* (CPUID registers, or MSRs) that it knows about. This is where it
becomes debatable, because a CPUID bit could be added without QEMU
knowing the corresponding MSR. In this case, the user probably expects
the MSR to have a nonzero. On one hand I agree that it would be
irresponsible, on the other hand that's the point of "-cpu
host,migratable=no".

If you want to proceed with the change, I don't have any problem with
considering it a QEMU bug that it doesn't copy over to the guest any
unknown leaves or MSRs.

Paolo

[1] Unfortunately it's not fatal because there are way way too many
models, and also because until recently TCG lacked AVX---and therefore
could only emulate completely some very old CPU models. But with "-cpu
Haswell,enforce" then everything's clean.