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Date: Mon, 18 Mar 2024 22:51:03 +0000
From: Mingwei Zhang <mizhang@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Namhyung Kim <namhyung@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH 3/3] KVM: VMX: Disable LBR virtualization if the CPU
 doesn't support LBR callstacks
Message-ID: <ZfjFV0W6-OCgugJl@google.com>
References: <20240307011344.835640-1-seanjc@google.com>
 <20240307011344.835640-4-seanjc@google.com>
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In-Reply-To: <20240307011344.835640-4-seanjc@google.com>

On Wed, Mar 06, 2024, Sean Christopherson wrote:
> Disable LBR virtualization if the CPU doesn't support callstacks, which
> were introduced in HSW (see commit e9d7f7cd97c4 ("perf/x86/intel: Add
> basic Haswell LBR call stack support"), as KVM unconditionally configures
> the perf LBR event with PERF_SAMPLE_BRANCH_CALL_STACK, i.e. LBR
> virtualization always fails on pre-HSW CPUs.
> 
> Simply disable LBR support on such CPUs, as it has never worked, i.e.
> there is no risk of breaking an existing setup, and figuring out a way
> to performantly context switch LBRs on old CPUs is not worth the effort.
> 
> Fixes: be635e34c284 ("KVM: vmx/pmu: Expose LBR_FMT in the MSR_IA32_PERF_CAPABILITIES")
> Cc: Mingwei Zhang <mizhang@google.com>
> Cc: Jim Mattson <jmattson@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Mingwei Zhang <mizhang@google.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 2a7cd66988a5..25a7652bee7c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7859,7 +7859,15 @@ static __init u64 vmx_get_perf_capabilities(void)
>  
>  	if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR)) {
>  		x86_perf_get_lbr(&vmx_lbr_caps);
> -		if (vmx_lbr_caps.nr)
> +
> +		/*
> +		 * KVM requires LBR callstack support, as the overhead due to
> +		 * context switching LBRs without said support is too high.
> +		 * See intel_pmu_create_guest_lbr_event() for more info.
> +		 */
> +		if (!vmx_lbr_caps.has_callstack)
> +			memset(&vmx_lbr_caps, 0, sizeof(vmx_lbr_caps));
> +		else if (vmx_lbr_caps.nr)
>  			perf_cap |= host_perf_cap & PMU_CAP_LBR_FMT;
>  	}
>  
> -- 
> 2.44.0.278.ge034bb2e1d-goog
>