On Thu, Jul 28, 2022, Like Xu wrote:
> On 28/7/2022 7:34 am, Sean Christopherson wrote:
> > Refresh the PMU if userspace modifies MSR_IA32_PERF_CAPABILITIES. KVM
> > consumes the vCPU's PERF_CAPABILITIES when enumerating PEBS support, but
> > relies on CPUID updates to refresh the PMU. I.e. KVM will do the wrong
> > thing if userspace stuffs PERF_CAPABILITIES _after_ setting guest CPUID.
>
> Unwise userspace should reap its consequences if it does not break KVM or host.
I don't think this is a case of userspace being weird or unwise. IMO, setting
CPUID before MSRs is perfectly logical and intuitive.
> When a guest feature can be defined/controlled by multiple KVM APIs entries,
> (such as SET_CPUID2, msr_feature, KVM_CAP, module_para), should KVM
> define the priority of these APIs (e.g. whether they can override each other) ?
KVM does have "rules" in the sense that it has an established ABI for things
like KVM_CAP and module params, though documentation may be lacking in some cases.
The CPUID and MSR ioctls don't have a prescribe ordering though.
> Removing this ambiguity ensures consistency in the architecture and behavior
> of all KVM features.
Agreed, but the CPUID and MSR ioctls (among many others) have existed for quite
some time. KVM likely can't retroactively force a specific order without breaking
one userspace or another.
> Any further performance optimizations can be based on these finalized values
> as you do.
>
> >
> > Opportunistically fix a curly-brace indentation.
> >
> > Fixes: c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS")
> > Cc: Like Xu <like.xu.linux@xxxxxxxxx>
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/x86.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 5366f884e9a7..362c538285db 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -3543,9 +3543,9 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > return 1;
> > vcpu->arch.perf_capabilities = data;
> > -
> > + kvm_pmu_refresh(vcpu);
>
> I had proposed this diff but was met with silence.
My apologies, I either missed it or didn't connect the dots.
