linux-kernel - Re: [PATCH v2] KVM: x86: omit absent pmu MSRs from MSR list

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Date:   Thu, 3 Oct 2019 10:20:38 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH v2] KVM: x86: omit absent pmu MSRs from MSR list

On Thu, Oct 3, 2019 at 3:10 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> INTEL_PMC_MAX_GENERIC is currently 32, which exceeds the 18 contiguous
> MSR indices reserved by Intel for event selectors.  Since some machines
> actually have MSRs past the reserved range, these may survive the
Not past, but *within* the reserved range.
> filtering of msrs_to_save array and would be rejected by KVM_GET/SET_MSR.
> To avoid this, cut the list to whatever CPUID reports for the host's
> architectural PMU.
>
> Reported-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Suggested-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Cc: Jim Mattson <jmattson@...gle.com>
> Fixes: e2ada66ec418 ("kvm: x86: Add Intel PMU MSRs to msrs_to_save[]", 2019-08-21)
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8072acaaf028..31607174f442 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5105,13 +5105,14 @@ long kvm_arch_vm_ioctl(struct file *filp,
>
>  static void kvm_init_msr_list(void)
>  {
> +       struct x86_pmu_capability x86_pmu;
>         u32 dummy[2];
>         unsigned i, j;
>
>         BUILD_BUG_ON_MSG(INTEL_PMC_MAX_FIXED != 4,
>                          "Please update the fixed PMCs in msrs_to_save[]");
> -       BUILD_BUG_ON_MSG(INTEL_PMC_MAX_GENERIC != 32,
> -                        "Please update the generic perfctr/eventsel MSRs in msrs_to_save[]");
> +
> +       perf_get_x86_pmu_capability(&x86_pmu);
>
>         for (i = j = 0; i < ARRAY_SIZE(msrs_to_save); i++) {
>                 if (rdmsr_safe(msrs_to_save[i], &dummy[0], &dummy[1]) < 0)
> @@ -5153,6 +5154,15 @@ static void kvm_init_msr_list(void)
>                                 intel_pt_validate_hw_cap(PT_CAP_num_address_ranges) * 2)
>                                 continue;
>                         break;
> +               case MSR_ARCH_PERFMON_PERFCTR0 ... MSR_ARCH_PERFMON_PERFCTR0 + 31:
You've truncated the list I originally provided, so I think this need
only go to MSR_ARCH_PERFMON_PERFCTR0 + 17. Otherwise, we could lose
some valuable MSRs.
> +                       if (msrs_to_save[i] - MSR_ARCH_PERFMON_PERFCTR0 >=
> +                           min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
Why involve INTEL_PMC_MAX_GENERIC here?
> +                               continue;
> +                       break;
> +               case MSR_ARCH_PERFMON_EVENTSEL0 ... MSR_ARCH_PERFMON_EVENTSEL0 + 31:
Same as the two comments above.
> +                       if (msrs_to_save[i] - MSR_ARCH_PERFMON_EVENTSEL0 >=
> +                           min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
> +                               continue;
>                 }
>                 default:
>                         break;
> --
> 1.8.3.1
>