Re: [PATCH v7 7/9] KVM: x86/pmu: Restrict RDPMC passthrough to known CPUs

[Jim Mattson]

On Wed, Jul 29, 2026 at 7:51 PM Mi, Dapeng <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:
>
>
> On 7/28/2026 3:21 AM, Zide Chen wrote:
> > RDPMC type encodings are vendor-defined and could theoretically be
> > extended on future CPUs. An unknown RDPMC type could leak host
> > PMU information to the guest through RDPMC passthrough.
> >
> > Rather than assuming future CPUs are safe, explicitly permit only
> > reviewed CPU models and require RDPMC interception everywhere else.
> > This ensures RDPMC passthrough is enabled only on CPUs whose RDPMC
> > encodings are audited and supported by KVM.
> >
> > Suggested-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> > Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
> > Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> > ---
> > v7: new patch.
> > ---
> >  arch/x86/kvm/pmu.c | 32 ++++++++++++++++++++++++++++++++
> >  1 file changed, 32 insertions(+)
> >
> > diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> > index 51c6d00a485f..092809bd757d 100644
> > --- a/arch/x86/kvm/pmu.c
> > +++ b/arch/x86/kvm/pmu.c
> > @@ -77,6 +77,21 @@ static const struct x86_cpu_id vmx_pebs_pdist_cpu[] = {
> >       {}
> >  };
> >
> > +/*
> > + * CPUs whose RDPMC encodings have been audited for KVM RDPMC
> > + * passthrough support.
> > + */
> > +static const struct x86_cpu_id kvm_rdpmc_known_cpus[] = {
> > +     X86_MATCH_VFM(INTEL_ATOM_DARKMONT_X, NULL),
> > +     X86_MATCH_VFM(INTEL_LUNARLAKE_M, NULL),
> > +     X86_MATCH_VFM(INTEL_NOVALAKE, NULL),
> > +     X86_MATCH_VFM(INTEL_NOVALAKE_L, NULL),
> > +     X86_MATCH_VFM(INTEL_PANTHERLAKE_L, NULL),
> > +     X86_MATCH_VFM(INTEL_PANTHERLAKE_R, NULL),
> > +     X86_MATCH_VFM(INTEL_WILDCATLAKE_L, NULL),
> > +     {}
> > +};
> > +
> >  /* NOTE:
> >   * - Each perf counter is defined as "struct kvm_pmc";
> >   * - There are two types of perf counters: general purpose (gp) and fixed.
> > @@ -807,6 +822,20 @@ bool kvm_need_perf_global_ctrl_intercept(struct kvm_vcpu *vcpu)
> >  }
> >  EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_need_perf_global_ctrl_intercept);
> >
> > +static bool kvm_rdpmc_encoding_supported(void)
> > +{
> > +     /* KVM understands all RDPMC encodings prior to PMU v6. */
> > +     if (kvm_host_pmu.version < 6)
>
> I'm not sure if this works for AMD platforms. Jim said current AMD
> platforms already have security holes and I believe the PMU version of AMD
> platforms are less than 6.

Let's not be pejorative! I said that AMD's RDPMC allows access to
counters that KVM does not (yet) virtualize, such as the data fabric
and L3 counters. The APM, volume 3, indicates that up to 28 counters
are supported today. KVM only virtualizes 6.

> Need AMD guys to confirm. @Sandipan, @Manali.