linux-kernel - Re: [RFC PATCH 18/41] KVM: x86/pmu: Intercept full-width GP counter MSRs by checking with perf capabilities

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Date: Thu, 11 Apr 2024 14:50:50 -0700
From: Jim Mattson <jmattson@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Xiong Zhang <xiong.y.zhang@...ux.intel.com>, pbonzini@...hat.com, 
	peterz@...radead.org, mizhang@...gle.com, kan.liang@...el.com, 
	zhenyuw@...ux.intel.com, dapeng1.mi@...ux.intel.com, kvm@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	zhiyuan.lv@...el.com, eranian@...gle.com, irogers@...gle.com, 
	samantha.alt@...el.com, like.xu.linux@...il.com, chao.gao@...el.com, 
	Xiong Zhang <xiong.y.zhang@...el.com>
Subject: Re: [RFC PATCH 18/41] KVM: x86/pmu: Intercept full-width GP counter
 MSRs by checking with perf capabilities

On Thu, Apr 11, 2024 at 2:23 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > From: Mingwei Zhang <mizhang@...gle.com>
> >
> > Intercept full-width GP counter MSRs in passthrough PMU if guest does not
> > have the capability to write in full-width. In addition, opportunistically
> > add a warning if non-full-width counter MSRs are also intercepted, in which
> > case it is a clear mistake.
> >
> > Co-developed-by: Xiong Zhang <xiong.y.zhang@...el.com>
> > Signed-off-by: Xiong Zhang <xiong.y.zhang@...el.com>
> > Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> > ---
> >  arch/x86/kvm/vmx/pmu_intel.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > index 7f6cabb2c378..49df154fbb5b 100644
> > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > @@ -429,6 +429,13 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >       default:
> >               if ((pmc = get_gp_pmc(pmu, msr, MSR_IA32_PERFCTR0)) ||
> >                   (pmc = get_gp_pmc(pmu, msr, MSR_IA32_PMC0))) {
> > +                     if (is_passthrough_pmu_enabled(vcpu) &&
> > +                         !(msr & MSR_PMC_FULL_WIDTH_BIT) &&
> > +                         !msr_info->host_initiated) {
> > +                             pr_warn_once("passthrough PMU never intercepts non-full-width PMU counters\n");
> > +                             return 1;
>
> This is broken, KVM must be prepared to handle WRMSR (and RDMSR and RDPMC) that
> come in through the emulator.

Don't tell me that we are still supporting CPUs that don't have
"unrestricted guest"! Sigh.