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Date:   Tue, 12 Dec 2023 18:25:37 -0800
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Subject: Re: [PATCH v9 10/28] KVM: x86/pmu: Explicitly check for RDPMC of
 unsupported Intel PMC types
From:   Sean Christopherson <seanjc@google.com>
To:     Jim Mattson <jmattson@google.com>
Cc:     Dapeng Mi <dapeng1.mi@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Kan Liang <kan.liang@linux.intel.com>,
        Jinrong Liang <cloudliang@tencent.com>,
        Aaron Lewis <aaronlewis@google.com>,
        Like Xu <likexu@tencent.com>
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On Mon, Dec 11, 2023, Jim Mattson wrote:
> On Mon, Dec 11, 2023 at 3:43=E2=80=AFPM Sean Christopherson <seanjc@googl=
e.com> wrote:
> > > > > @@ -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.  Not=
e, 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 coun=
ter.
> > > > > +      * i.e. let RDPMC fail due to accessing a non-existent coun=
ter.  Reject
> > > > > +      * attempts to read all other types, which are unknown/unsu=
pported.
> > > > >        */
> > > > > -     idx &=3D ~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] =E2=89=A0 0), ECX[31:16] specifies type of PMC wh=
ile
> > > 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?
>=20
> You're right. The code is fine. My brain is not.
>=20
> But what's wrong with something like:
>=20
> type =3D idx & INTEL_RDPMC_TYPE_MASK;
> if (type !=3D INTEL_RDPMC_GP && type !=3D INTEL_RDPMC_FIXED) ...
>=20
> This makes it more clear what kvm accepts and what it doesn't accept,
> regardless of the actual values of the macros.

Because when I read the SDM, my reading was heavily colored by KVM's existi=
ng
implementation.  And the SDM using 4000H and 2000H for the non-zero types d=
oesn't
help (those scream "flags" to me).  But rereading things, the SDM clearly s=
tates
they are explicit, distinct types.  I'll massage this to have KVM treat the=
m as
such.