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

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

[Wanpeng Li]

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

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

[Sean Christopherson]

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
> 

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

[Wanpeng Li]

On Tue, 11 May 2021 at 08:18, Sean Christopherson <seanjc@google.com> wrote:
>
> 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,

Will send out a new version after 1-2 get reviewed. :)

>
> Reviewed-by: Sean Christopherson <seanjc@google.com>
>
> >
> >       vcpu->arch.st.preempted = 0;
> >
> > --
> > 2.7.4
> >