Re: [PATCH v9 10/28] KVM: x86/pmu: Explicitly check for RDPMC of unsupported Intel PMC types

[Sean Christopherson <seanjc@google.com>]

On Mon, Dec 11, 2023, Jim Mattson wrote:
> On Sun, Dec 10, 2023 at 10:26 PM Mi, Dapeng <dapeng1.mi@linux.intel.com> wrote:
> >
> >
> > On 12/2/2023 8:03 AM, Sean Christopherson wrote:
> > > Explicitly check for attempts to read unsupported PMC types instead of
> > > letting the bounds check fail.  Functionally, letting the check fail is
> > > ok, but it's unnecessarily subtle and does a poor job of documenting the
> > > architectural behavior that KVM is emulating.
> > >
> > > Opportunistically add macros for the type vs. index to further document
> > > what is going on.
> > >
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
> > >   arch/x86/kvm/vmx/pmu_intel.c | 11 +++++++++--
> > >   1 file changed, 9 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > > index 644de27bd48a..bd4f4bdf5419 100644
> > > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > > @@ -23,6 +23,9 @@
> > >   /* Perf's "BASE" is wildly misleading, this is a single-bit flag, not a base. */
> > >   #define INTEL_RDPMC_FIXED   INTEL_PMC_FIXED_RDPMC_BASE
> > >
> > > +#define INTEL_RDPMC_TYPE_MASK        GENMASK(31, 16)
> > > +#define INTEL_RDPMC_INDEX_MASK       GENMASK(15, 0)
> > > +
> > >   #define MSR_PMC_FULL_WIDTH_BIT      (MSR_IA32_PMC0 - MSR_IA32_PERFCTR0)
> > >
> > >   static void reprogram_fixed_counters(struct kvm_pmu *pmu, u64 data)
> > > @@ -82,9 +85,13 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu,
> > >       /*
> > >        * Fixed PMCs are supported on all architectural PMUs.  Note, KVM only
> > >        * emulates fixed PMCs for PMU v2+, but the flag itself is still valid,
> > > -      * i.e. let RDPMC fail due to accessing a non-existent counter.
> > > +      * i.e. let RDPMC fail due to accessing a non-existent counter.  Reject
> > > +      * attempts to read all other types, which are unknown/unsupported.
> > >        */
> > > -     idx &= ~INTEL_RDPMC_FIXED;
> > > +     if (idx & INTEL_RDPMC_TYPE_MASK & ~INTEL_RDPMC_FIXED)
> 
> You know how I hate to be pedantic (ROFL), but the SDM only says:
> 
> If the processor does support architectural performance monitoring
> (CPUID.0AH:EAX[7:0] ≠ 0), ECX[31:16] specifies type of PMC while
> ECX[15:0] specifies the index of the PMC to be read within that type.
> 
> It does not say that the types are bitwise-exclusive.
> 
> Yes, the types defined thus far are bitwise-exclusive, but who knows
> what tomorrow may bring?

The goal isn't to make the types exclusive, the goal is to reject types that
aren't supported by KVM.  The above accomplishes that, no?  I don't see how KVM
could get a false negative or false positive, the above allows exactly FIXED and
"none" types.  Or are you objecting to the comment?