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 18:28:51 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <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>,
        "# v3 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH 3/3] KVM: X86: Fix vCPU preempted state from guest point
 of view

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

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

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