Re: [PATCH v3 2/3] KVM: x86/pmu: Limit the maximum number of supported Intel GP counters

[Sean Christopherson]

On Mon, Sep 19, 2022, Like Xu wrote:
> From: Like Xu <likexu@xxxxxxxxxxx>
> 
> The Intel Architectural IA32_PMCx MSRs addresses range allows for
> a maximum of 8 GP counters. A local macro (named KVM_INTEL_PMC_MAX_GENERIC)
> is introduced to take back control of this virtual capability to avoid
> errors introduced by the out-of-bound counter emulations.

Phrase changelogs as commands.

> Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
> Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  6 +++++-
>  arch/x86/kvm/pmu.c              |  2 +-
>  arch/x86/kvm/vmx/pmu_intel.c    |  4 ++--
>  arch/x86/kvm/x86.c              | 12 +++++++-----
>  4 files changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 2c96c43c313a..17abcf5c496a 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -501,6 +501,10 @@ struct kvm_pmc {
>  	bool intr;
>  };
>  
> +/* More counters may conflict with other existing Architectural MSRs */
> +#define KVM_INTEL_PMC_MAX_GENERIC	8

This is weird and backwards.  Common x86 code shouldn't "prefer" Intel over AMD,
or vice versa.  Similar to KVM_MAX_NR_USER_RETURN_MSRS, the way to do this is to
define KVM's common software limit, and then verify that the vendor limits are
below that common limit.  E.g.

#define KVM_MAX_NR_PMU_GP_COUNTERS	8

and then add compile-time assertions that Intel stays below the max (and obviously
AMD as well).

> +#define MSR_ARCH_PERFMON_PERFCTR_MAX	(MSR_ARCH_PERFMON_PERFCTR0 + KVM_INTEL_PMC_MAX_GENERIC - 1)
> +#define MSR_ARCH_PERFMON_EVENTSEL_MAX	(MSR_ARCH_PERFMON_EVENTSEL0 + KVM_INTEL_PMC_MAX_GENERIC - 1)

These are Intel specific, correct?  I.e. "arch" means "Intel architectural MSRs"?

The perf-defined names are out of KVM's control, but adding what appears to be
generic #defines in common KVM that are actually Intel specific is confusing.
Given that there's only a single user, I think the easiest thing is to just open
code the users, e.g.

		case MSR_ARCH_PERFMON_PERFCTR0 ...
		     MSR_ARCH_PERFMON_PERFCTR0 + KVM_MAX_NR_PMU_GP_COUNTERS - 1:
			if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_PERFCTR0 >=
			    min(KVM_MAX_NR_PMU_GP_COUNTERS, kvm_pmu_cap.num_counters_gp))
				continue;
			break;
		case MSR_ARCH_PERFMON_EVENTSEL0 ...
		     MSR_ARCH_PERFMON_EVENTSEL0 + KVM_MAX_NR_PMU_GP_COUNTERS - 1:
			if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >=
			    min(KVM_INTEL_PMC_MAX_GENERIC, kvm_pmu_cap.num_counters_gp))
				continue;
			break;