linux-kernel - Re: [PATCH 3/3] KVM: X86: Fix vCPU preempted state from guest point of view

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Date:   Tue, 11 May 2021 00:18:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, stable@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: X86: Fix vCPU preempted state from guest point
 of view

On Sat, May 08, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> Commit 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's 
> CPUID) avoids to access pv tlb shootdown host side logic when this pv feature 
> is not exposed to guest, however, kvm_steal_time.preempted not only leveraged 
> by pv tlb shootdown logic but also mitigate the lock holder preemption issue. 
> From guest point of view, vCPU is always preempted since we lose the reset of
> kvm_steal_time.preempted before vmentry if pv tlb shootdown feature is not 
> exposed. This patch fixes it by clearing kvm_steal_time.preempted before 
> vmentry.
> 
> Fixes: 66570e966dd9 (kvm: x86: only provide PV features if enabled in guest's CPUID)
> Cc: stable@...r.kernel.org
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/x86.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c0244a6..c38e990 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3105,7 +3105,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>  				       st->preempted & KVM_VCPU_FLUSH_TLB);
>  		if (xchg(&st->preempted, 0) & KVM_VCPU_FLUSH_TLB)
>  			kvm_vcpu_flush_tlb_guest(vcpu);
> -	}
> +	} else
> +		st->preempted = 0;

Curly braces needed since the if-statment needs 'em.  Other than that,

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

>  
>  	vcpu->arch.st.preempted = 0;
>  
> -- 
> 2.7.4
>