Re: [PATCH] KVM: vPMU: Use atomic bit operations for global_status

[Mingwei Zhang]

On Mon, Sep 11, 2023 at 8:01 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Mon, Sep 11, 2023, Mingwei Zhang wrote:
> > Use atomic bit operations for pmu->global_status because it may suffer from
> > race conditions between emulated overflow in KVM vPMU and PEBS overflow in
> > host PMI handler.
>
> Only if the host PMI occurs on a different pCPU, and if that can happen don't we
> have a much larger problem?

Why on different pCPU?  For vPMU, I think there is always contention
between the vCPU thread and the host PMI handler running on the same
pCPU, no?

So, in that case, anything that __kvm_perf_overflow(..., in_pmi=true)
touches on struct kvm_pmu will potentially race with the functions
like reprogram_counter() -> __kvm_perf_overflow(..., in_pmi=false).

-Mingwei
>
> > Fixes: f331601c65ad ("KVM: x86/pmu: Don't generate PEBS records for emulated instructions")
> > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/pmu.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index edb89b51b383..00b48f25afdb 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -117,11 +117,11 @@ static inline void __kvm_perf_overflow(struct kvm_pmc *pmc, bool in_pmi)
> >                       skip_pmi = true;
> >               } else {
> >                       /* Indicate PEBS overflow PMI to guest. */
> > -                     skip_pmi = __test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT,
> > -                                                   (unsigned long *)&pmu->global_status);
> > +                     skip_pmi = test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT,
> > +                                                 (unsigned long *)&pmu->global_status);
> >               }
> >       } else {
> > -             __set_bit(pmc->idx, (unsigned long *)&pmu->global_status);
> > +             set_bit(pmc->idx, (unsigned long *)&pmu->global_status);
> >       }
> >
> >       if (!pmc->intr || skip_pmi)
> >
> > base-commit: e2013f46ee2e721567783557c301e5c91d0b74ff
> > --
> > 2.42.0.283.g2d96d420d3-goog
> >