linux-kernel - Re: [PATCH] KVM: vPMU: Use atomic bit operations for global

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Date:   Mon, 11 Sep 2023 08:01:44 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Dapeng Mi <dapeng1.mi@...el.com>,
        Jim Mattson <jmattson@...gle.com>, Like Xu <likexu@...cent.com>
Subject: Re: [PATCH] KVM: vPMU: Use atomic bit operations for global_status

On Mon, Sep 11, 2023, Mingwei Zhang wrote:
> Use atomic bit operations for pmu->global_status because it may suffer from
> race conditions between emulated overflow in KVM vPMU and PEBS overflow in
> host PMI handler.

Only if the host PMI occurs on a different pCPU, and if that can happen don't we
have a much larger problem?

> Fixes: f331601c65ad ("KVM: x86/pmu: Don't generate PEBS records for emulated instructions")
> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> ---
>  arch/x86/kvm/pmu.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index edb89b51b383..00b48f25afdb 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -117,11 +117,11 @@ static inline void __kvm_perf_overflow(struct kvm_pmc *pmc, bool in_pmi)
>  			skip_pmi = true;
>  		} else {
>  			/* Indicate PEBS overflow PMI to guest. */
> -			skip_pmi = __test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT,
> -						      (unsigned long *)&pmu->global_status);
> +			skip_pmi = test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT,
> +						    (unsigned long *)&pmu->global_status);
>  		}
>  	} else {
> -		__set_bit(pmc->idx, (unsigned long *)&pmu->global_status);
> +		set_bit(pmc->idx, (unsigned long *)&pmu->global_status);
>  	}
>  
>  	if (!pmc->intr || skip_pmi)
> 
> base-commit: e2013f46ee2e721567783557c301e5c91d0b74ff
> -- 
> 2.42.0.283.g2d96d420d3-goog
>