linux-kernel - Re: [PATCH 1/4] KVM: x86/svm/pmu: Limit the maximum number of supported GP counters

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Date:   Mon, 5 Sep 2022 10:26:45 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sandipan Das <sandipan.das@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM: x86/svm/pmu: Limit the maximum number of
 supported GP counters

On Mon, Sep 5, 2022 at 5:45 AM Like Xu <like.xu.linux@...il.com> wrote:
>
> From: Like Xu <likexu@...cent.com>
>
> The AMD PerfMonV2 specification allows for a maximum of 16 GP counters,
> which is clearly not supported with zero code effort in the current KVM.
>
> A local macro (named like INTEL_PMC_MAX_GENERIC) is introduced to
> take back control of this virt capability, which also makes it easier to
> statically partition all available counters between hosts and guests.
>
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/kvm/pmu.h     | 2 ++
>  arch/x86/kvm/svm/pmu.c | 7 ++++---
>  arch/x86/kvm/x86.c     | 2 ++
>  3 files changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index 847e7112a5d3..e3a3813b6a38 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -18,6 +18,8 @@
>  #define VMWARE_BACKDOOR_PMC_REAL_TIME          0x10001
>  #define VMWARE_BACKDOOR_PMC_APPARENT_TIME      0x10002
>
> +#define KVM_AMD_PMC_MAX_GENERIC        AMD64_NUM_COUNTERS_CORE
> +
>  struct kvm_event_hw_type_mapping {
>         u8 eventsel;
>         u8 unit_mask;
> diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> index 2ec420b85d6a..f99f2c869664 100644
> --- a/arch/x86/kvm/svm/pmu.c
> +++ b/arch/x86/kvm/svm/pmu.c
> @@ -192,9 +192,10 @@ static void amd_pmu_init(struct kvm_vcpu *vcpu)
>         struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>         int i;
>
> -       BUILD_BUG_ON(AMD64_NUM_COUNTERS_CORE > INTEL_PMC_MAX_GENERIC);
> +       BUILD_BUG_ON(AMD64_NUM_COUNTERS_CORE > KVM_AMD_PMC_MAX_GENERIC);
> +       BUILD_BUG_ON(KVM_AMD_PMC_MAX_GENERIC > INTEL_PMC_MAX_GENERIC);
>
> -       for (i = 0; i < AMD64_NUM_COUNTERS_CORE ; i++) {
> +       for (i = 0; i < KVM_AMD_PMC_MAX_GENERIC ; i++) {
>                 pmu->gp_counters[i].type = KVM_PMC_GP;
>                 pmu->gp_counters[i].vcpu = vcpu;
>                 pmu->gp_counters[i].idx = i;
> @@ -207,7 +208,7 @@ static void amd_pmu_reset(struct kvm_vcpu *vcpu)
>         struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
>         int i;
>
> -       for (i = 0; i < AMD64_NUM_COUNTERS_CORE; i++) {
> +       for (i = 0; i < KVM_AMD_PMC_MAX_GENERIC; i++) {
>                 struct kvm_pmc *pmc = &pmu->gp_counters[i];
>
>                 pmc_stop_counter(pmc);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 43a6a7efc6ec..b9738efd8425 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1444,12 +1444,14 @@ static const u32 msrs_to_save_all[] = {
>         MSR_ARCH_PERFMON_EVENTSEL0 + 16, MSR_ARCH_PERFMON_EVENTSEL0 + 17,

IIRC, the effective maximum on the Intel side is 18, despite what
INTEL_PMC_MAX_GENERIC says, due to collisions with other existing MSR
indices. That's why the Intel list stops here. Should we introduce a
KVM_INTEL_PMC_MAX_GENERIC as well?

>         MSR_IA32_PEBS_ENABLE, MSR_IA32_DS_AREA, MSR_PEBS_DATA_CFG,
>
> +       /* This part of MSRs should match KVM_AMD_PMC_MAX_GENERIC. */

Perhaps the comment above should be moved down two lines, since the
next two lines deal with the legacy counters.

>         MSR_K7_EVNTSEL0, MSR_K7_EVNTSEL1, MSR_K7_EVNTSEL2, MSR_K7_EVNTSEL3,
>         MSR_K7_PERFCTR0, MSR_K7_PERFCTR1, MSR_K7_PERFCTR2, MSR_K7_PERFCTR3,
>         MSR_F15H_PERF_CTL0, MSR_F15H_PERF_CTL1, MSR_F15H_PERF_CTL2,
>         MSR_F15H_PERF_CTL3, MSR_F15H_PERF_CTL4, MSR_F15H_PERF_CTL5,
>         MSR_F15H_PERF_CTR0, MSR_F15H_PERF_CTR1, MSR_F15H_PERF_CTR2,
>         MSR_F15H_PERF_CTR3, MSR_F15H_PERF_CTR4, MSR_F15H_PERF_CTR5,

At some point, we may want to consider populating the PMU MSR list
dynamically, rather than statically enumerating all of them (for both
AMD and Intel).

Reviewed-by: Jim Mattson <jmattson@...gle.com>