Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU state for Intel CPU

[Jim Mattson]

On Thu, Apr 11, 2024 at 2:44 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > From: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> >
> > Implement the save/restore of PMU state for pasthrough PMU in Intel. In
> > passthrough mode, KVM owns exclusively the PMU HW when control flow goes to
> > the scope of passthrough PMU. Thus, KVM needs to save the host PMU state
> > and gains the full HW PMU ownership. On the contrary, host regains the
> > ownership of PMU HW from KVM when control flow leaves the scope of
> > passthrough PMU.
> >
> > Implement PMU context switches for Intel CPUs and opptunistically use
> > rdpmcl() instead of rdmsrl() when reading counters since the former has
> > lower latency in Intel CPUs.
> >
> > Co-developed-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> > Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/vmx/pmu_intel.c | 73 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 73 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > index 0d58fe7d243e..f79bebe7093d 100644
> > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > @@ -823,10 +823,83 @@ void intel_passthrough_pmu_msrs(struct kvm_vcpu *vcpu)
> >
> >  static void intel_save_pmu_context(struct kvm_vcpu *vcpu)
>
> I would prefer there be a "guest" in there somewhere, e.g. intel_save_guest_pmu_context().
>
> >  {
> > +     struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> > +     struct kvm_pmc *pmc;
> > +     u32 i;
> > +
> > +     if (pmu->version != 2) {
> > +             pr_warn("only PerfMon v2 is supported for passthrough PMU");
> > +             return;
> > +     }
> > +
> > +     /* Global ctrl register is already saved at VM-exit. */
> > +     rdmsrl(MSR_CORE_PERF_GLOBAL_STATUS, pmu->global_status);
> > +     /* Clear hardware MSR_CORE_PERF_GLOBAL_STATUS MSR, if non-zero. */
> > +     if (pmu->global_status)
> > +             wrmsrl(MSR_CORE_PERF_GLOBAL_OVF_CTRL, pmu->global_status);
> > +
> > +     for (i = 0; i < pmu->nr_arch_gp_counters; i++) {
> > +             pmc = &pmu->gp_counters[i];
> > +             rdpmcl(i, pmc->counter);
> > +             rdmsrl(i + MSR_ARCH_PERFMON_EVENTSEL0, pmc->eventsel);
> > +             /*
> > +              * Clear hardware PERFMON_EVENTSELx and its counter to avoid
> > +              * leakage and also avoid this guest GP counter get accidentally
> > +              * enabled during host running when host enable global ctrl.
> > +              */
> > +             if (pmc->eventsel)
> > +                     wrmsrl(MSR_ARCH_PERFMON_EVENTSEL0 + i, 0);
> > +             if (pmc->counter)
> > +                     wrmsrl(MSR_IA32_PMC0 + i, 0);
>
> This doesn't make much sense.  The kernel already has full access to the guest,
> I don't see what is gained by zeroing out the MSRs just to hide them from perf.
>
> Similarly, if perf enables a counter if PERF_GLOBAL_CTRL without first restoring
> the event selector, we gots problems.
>
> Same thing for the fixed counters below.  Can't this just be?
>
>         for (i = 0; i < pmu->nr_arch_gp_counters; i++)
>                 rdpmcl(i, pmu->gp_counters[i].counter);
>
>         for (i = 0; i < pmu->nr_arch_fixed_counters; i++)
>                 rdpmcl(INTEL_PMC_FIXED_RDPMC_BASE | i,
>                        pmu->fixed_counters[i].counter);
>
> > +     }
> > +
> > +     rdmsrl(MSR_CORE_PERF_FIXED_CTR_CTRL, pmu->fixed_ctr_ctrl);
> > +     /*
> > +      * Clear hardware FIXED_CTR_CTRL MSR to avoid information leakage and
> > +      * also avoid these guest fixed counters get accidentially enabled
> > +      * during host running when host enable global ctrl.
> > +      */
> > +     if (pmu->fixed_ctr_ctrl)
> > +             wrmsrl(MSR_CORE_PERF_FIXED_CTR_CTRL, 0);
> > +     for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
> > +             pmc = &pmu->fixed_counters[i];
> > +             rdpmcl(INTEL_PMC_FIXED_RDPMC_BASE | i, pmc->counter);
> > +             if (pmc->counter)
> > +                     wrmsrl(MSR_CORE_PERF_FIXED_CTR0 + i, 0);
> > +     }
> >  }
> >
> >  static void intel_restore_pmu_context(struct kvm_vcpu *vcpu)
> >  {
> > +     struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> > +     struct kvm_pmc *pmc;
> > +     u64 global_status;
> > +     int i;
> > +
> > +     if (pmu->version != 2) {
> > +             pr_warn("only PerfMon v2 is supported for passthrough PMU");
> > +             return;
> > +     }
> > +
> > +     /* Clear host global_ctrl and global_status MSR if non-zero. */
> > +     wrmsrl(MSR_CORE_PERF_GLOBAL_CTRL, 0);
>
> Why?  PERF_GLOBAL_CTRL will be auto-loaded at VM-Enter, why do it now?
>
> > +     rdmsrl(MSR_CORE_PERF_GLOBAL_STATUS, global_status);
> > +     if (global_status)
> > +             wrmsrl(MSR_CORE_PERF_GLOBAL_OVF_CTRL, global_status);
>
> This seems especially silly, isn't the full MSR being written below?  Or am I
> misunderstanding how these things work?

LOL! You expect CPU design to follow basic logic?!?

Writing a 1 to a bit in IA32_PERF_GLOBAL_STATUS_SET sets the
corresponding bit in IA32_PERF_GLOBAL_STATUS to 1.

Writing a 0 to a bit in to IA32_PERF_GLOBAL_STATUS_SET is a nop.

To clear a bit in IA32_PERF_GLOBAL_STATUS, you need to write a 1 to
the corresponding bit in IA32_PERF_GLOBAL_STATUS_RESET (aka
IA32_PERF_GLOBAL_OVF_CTRL).

> > +     wrmsrl(MSR_CORE_PERF_GLOBAL_STATUS_SET, pmu->global_status);
> > +
> > +     for (i = 0; i < pmu->nr_arch_gp_counters; i++) {
> > +             pmc = &pmu->gp_counters[i];
> > +             wrmsrl(MSR_IA32_PMC0 + i, pmc->counter);
> > +             wrmsrl(MSR_ARCH_PERFMON_EVENTSEL0 + i, pmc->eventsel);
> > +     }
> > +
> > +     wrmsrl(MSR_CORE_PERF_FIXED_CTR_CTRL, pmu->fixed_ctr_ctrl);
> > +     for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
> > +             pmc = &pmu->fixed_counters[i];
> > +             wrmsrl(MSR_CORE_PERF_FIXED_CTR0 + i, pmc->counter);
> > +     }
> >  }
> >
> >  struct kvm_pmu_ops intel_pmu_ops __initdata = {
> > --
> > 2.34.1
> >