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    SubjectRe: [RFC PATCH 26/41] KVM: x86/pmu: Add host_perf_cap field in kvm_caps to record host PMU capability
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    On Fri, Jan 26, 2024, Xiong Zhang wrote:
    > From: Mingwei Zhang <mizhang@google.com>
    >
    > Add host_perf_cap field in kvm_caps to record host PMU capability. This
    > helps KVM recognize the PMU capability difference between host and guest.
    > This awareness improves performance in PMU context switch. In particular,
    > KVM will need to zero out all MSRs that guest PMU does not use but host PMU
    > does use. Having the host PMU feature set cached in host_perf_cap in
    > kvm_caps structure saves a rdmsrl() to IA32_PERF_CAPABILITY MSR on each PMU
    > context switch. In addition, this is more convenient approach than open
    > another API on the host perf subsystem side.
    >
    > Signed-off-by: Mingwei Zhang <mizhang@google.com>
    > ---
    > arch/x86/kvm/vmx/vmx.c | 17 +++++++++--------
    > arch/x86/kvm/x86.h | 1 +
    > 2 files changed, 10 insertions(+), 8 deletions(-)
    >
    > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
    > index 349954f90fe9..50100954cd92 100644
    > --- a/arch/x86/kvm/vmx/vmx.c
    > +++ b/arch/x86/kvm/vmx/vmx.c
    > @@ -7896,32 +7896,33 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
    > vmx_update_exception_bitmap(vcpu);
    > }
    >
    > -static u64 vmx_get_perf_capabilities(void)
    > +static void vmx_get_perf_capabilities(void)
    > {
    > u64 perf_cap = PMU_CAP_FW_WRITES;
    > struct x86_pmu_lbr lbr;
    > - u64 host_perf_cap = 0;
    > +
    > + kvm_caps.host_perf_cap = 0;
    >
    > if (!enable_pmu)
    > - return 0;
    > + return;
    >
    > if (boot_cpu_has(X86_FEATURE_PDCM))
    > - rdmsrl(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);
    > + rdmsrl(MSR_IA32_PERF_CAPABILITIES, kvm_caps.host_perf_cap);

    I would strongly prefer KVM snapshot the host's MSR_IA32_PERF_CAPABILITIES, if
    the CPU has PDMC, i.e. not leave it zero if the PMU is disabled.

    >
    > if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR) &&
    > !enable_passthrough_pmu) {
    > x86_perf_get_lbr(&lbr);
    > if (lbr.nr)
    > - perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
    > + perf_cap |= kvm_caps.host_perf_cap & PMU_CAP_LBR_FMT;
    > }
    >
    > if (vmx_pebs_supported() && !enable_passthrough_pmu) {
    > - perf_cap |= host_perf_cap & PERF_CAP_PEBS_MASK;
    > + perf_cap |= kvm_caps.host_perf_cap & PERF_CAP_PEBS_MASK;
    > if ((perf_cap & PERF_CAP_PEBS_FORMAT) < 4)
    > perf_cap &= ~PERF_CAP_PEBS_BASELINE;
    > }
    >
    > - return perf_cap;
    > + kvm_caps.supported_perf_cap = perf_cap;
    > }
    >
    > static __init void vmx_set_cpu_caps(void)
    > @@ -7946,7 +7947,7 @@ static __init void vmx_set_cpu_caps(void)
    >
    > if (!enable_pmu)
    > kvm_cpu_cap_clear(X86_FEATURE_PDCM);
    > - kvm_caps.supported_perf_cap = vmx_get_perf_capabilities();
    > + vmx_get_perf_capabilities();
    >
    > if (!enable_sgx) {
    > kvm_cpu_cap_clear(X86_FEATURE_SGX);
    > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
    > index 38b73e98eae9..a29eb0469d7e 100644
    > --- a/arch/x86/kvm/x86.h
    > +++ b/arch/x86/kvm/x86.h
    > @@ -28,6 +28,7 @@ struct kvm_caps {
    > u64 supported_mce_cap;
    > u64 supported_xcr0;
    > u64 supported_xss;
    > + u64 host_perf_cap;
    > u64 supported_perf_cap;

    This is confusing, host_perf_cap doesn't track "capabilities" so much as it tracks
    a raw host value. Luckily, I have a series that I am going to post this week
    that adds another struct for tracking host values, e.g. host_xss, host_efer, etc.

    > };
    >
    > --
    > 2.34.1
    >

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