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Message-ID: <CALMp9eS4H-WZXRCrp+6aAgwAp+qP2BgJx5ik5kA7vdyQ9qzARg@mail.gmail.com>
Date: Thu, 11 Apr 2024 15:19:41 -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
Subject: Re: [RFC PATCH 23/41] KVM: x86/pmu: Implement the save/restore of PMU
state for Intel CPU
On Thu, Apr 11, 2024 at 2:44 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > From: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> >
> > 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@...gle.com>
> > Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> > Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> > ---
> > 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
> >
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