linux-kernel - Re: [PATCH 07/15] KVM: Use dedicated flag to track if KVM is handling an NMI from guest

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Message-ID: <YSiUnDbi/aZ3nunT@hirez.programming.kicks-ass.net>
Date:   Fri, 27 Aug 2021 09:30:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Guo Ren <guoren@...nel.org>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-csky@...r.kernel.org, linux-riscv@...ts.infradead.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Artem Kashkanov <artem.kashkanov@...el.com>,
        Like Xu <like.xu.linux@...il.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>
Subject: Re: [PATCH 07/15] KVM: Use dedicated flag to track if KVM is
 handling an NMI from guest

On Thu, Aug 26, 2021 at 05:57:10PM -0700, Sean Christopherson wrote:
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 5cedc0e8a5d5..4c5ba4128b38 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -395,9 +395,10 @@ static inline void kvm_unregister_perf_callbacks(void)
>  
>  DECLARE_PER_CPU(struct kvm_vcpu *, current_vcpu);
>  
> -static inline void kvm_before_interrupt(struct kvm_vcpu *vcpu)
> +static inline void kvm_before_interrupt(struct kvm_vcpu *vcpu, bool is_nmi)
>  {
>  	__this_cpu_write(current_vcpu, vcpu);
> +	WRITE_ONCE(vcpu->arch.handling_nmi_from_guest, is_nmi);
>  
>  	kvm_register_perf_callbacks();
>  }
> @@ -406,6 +407,7 @@ static inline void kvm_after_interrupt(struct kvm_vcpu *vcpu)
>  {
>  	kvm_unregister_perf_callbacks();
>  
> +	WRITE_ONCE(vcpu->arch.handling_nmi_from_guest, false);
>  	__this_cpu_write(current_vcpu, NULL);
>  }

Does this rely on kvm_{,un}register_perf_callback() being a function
call and thus implying a sequence point to order the stores?