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

[Sean Christopherson <seanjc@google.com>]

On Sat, May 08, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.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@vger.kernel.org
> Signed-off-by: Wanpeng Li <wanpengli@tencent.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@google.com>

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