Re: [PATCH v4 09/12] KVM: x86/pmu: Forget PERFCTR_CORE if the min num of counters isn't met

From: Sean Christopherson <seanjc@google.com>
To: Like Xu <like.xu.linux@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 09/12] KVM: x86/pmu: Forget PERFCTR_CORE if the min num of counters isn't met
Date: Thu, 6 Apr 2023 17:32:05 -0700	[thread overview]
Message-ID: <ZC9khVlFr2RYd83P@google.com> (raw)
In-Reply-To: <20230214050757.9623-10-likexu@tencent.com>

On Tue, Feb 14, 2023, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
> 
> KVM should sanity check the number of counters enumerated by perf and
> explicitly drop PERFCTR_CORE support if the min isn't met. E.g. if KVM
> needs 6 counters and perf says there are 4, then something is wrong and
> enumerating 6 to the guest is only going to cause more issues.
> 
> Opportunistically, the existing kvm_cpu_cap_check_and_set() makes it
> easier to simplify the host check before setting the PERFCTR_CORE flag.

Once again, state what the patch does.

> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Like Xu <likexu@tencent.com>
> ---
>  arch/x86/kvm/svm/svm.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index dd21e8b1a259..f4a4691b4f4e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4910,9 +4910,14 @@ static __init void svm_set_cpu_caps(void)
>  	    boot_cpu_has(X86_FEATURE_AMD_SSBD))
>  		kvm_cpu_cap_set(X86_FEATURE_VIRT_SSBD);
>  
> -	/* AMD PMU PERFCTR_CORE CPUID */
> -	if (enable_pmu && boot_cpu_has(X86_FEATURE_PERFCTR_CORE))
> -		kvm_cpu_cap_set(X86_FEATURE_PERFCTR_CORE);
> +	if (enable_pmu) {

A comment would be very helpful here.   Even better would be if we can convince
perf to rename AMD64_NUM_COUNTERS to AMD64_LEGACY_NUM_COUNTERS.

> +		if (kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS_CORE) {
> +			kvm_pmu_cap.num_counters_gp = AMD64_NUM_COUNTERS;

Hmm, this subtly relies on kvm_init_pmu_capability() running first.  That
_probably_ won't change, but like the arch LBRs side of things I would prefer to
avoid taking unnecessarily.  E.g. something like this?

		/*
		 * Enumerate support for PERFCTR_CORE if and only if KVM has
		 * access to enough counters to virtualize "core" support,
		 * otherwise limit vPMU support to the legacy number of counters.
		 */
		if (kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS_CORE)
			kvm_pmu_cap.num_counters_gp = min(AMD64_NUM_COUNTERS,
							  kvm_pmu_cap.num_counters_gp);

It's a bit paranoid, but practically speaking it doesn't cost us anything.

> +		} else {
> +			/* AMD PMU PERFCTR_CORE CPUID */

Meh, this is not a very helpful comment, no need to carry it forward.  And if you
drop it, then the need for curly braces goes away.

> +			kvm_cpu_cap_check_and_set(X86_FEATURE_PERFCTR_CORE);
> +		}
> +	}
>  
>  	/* CPUID 0x8000001F (SME/SEV features) */
>  	sev_set_cpu_caps();
> -- 
> 2.39.1
> 