linux-kernel - Re: [PATCH 12/13] KVM: x86: Replace late check_nested

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Message-ID: <20200428222010.GN12735@linux.intel.com>
Date:   Tue, 28 Apr 2020 15:20:10 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH 12/13] KVM: x86: Replace late check_nested_events() hack
 with more precise fix

On Tue, Apr 28, 2020 at 03:12:51PM -0700, Jim Mattson wrote:
> On Wed, Apr 22, 2020 at 7:26 PM Sean Christopherson
> <sean.j.christopherson@...el.com> wrote:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 7c49a7dc601f..d9d6028a77e0 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7755,24 +7755,10 @@ static int inject_pending_event(struct kvm_vcpu *vcpu)
> >                 --vcpu->arch.nmi_pending;
> >                 vcpu->arch.nmi_injected = true;
> >                 kvm_x86_ops.set_nmi(vcpu);
> > -       } else if (kvm_cpu_has_injectable_intr(vcpu)) {
> > -               /*
> > -                * Because interrupts can be injected asynchronously, we are
> > -                * calling check_nested_events again here to avoid a race condition.
> > -                * See https://lkml.org/lkml/2014/7/2/60 for discussion about this
> > -                * proposal and current concerns.  Perhaps we should be setting
> > -                * KVM_REQ_EVENT only on certain events and not unconditionally?
> > -                */
> > -               if (is_guest_mode(vcpu) && kvm_x86_ops.check_nested_events) {
> > -                       r = kvm_x86_ops.check_nested_events(vcpu);
> > -                       if (r != 0)
> > -                               return r;
> > -               }
> > -               if (kvm_x86_ops.interrupt_allowed(vcpu)) {
> > -                       kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu),
> > -                                           false);
> > -                       kvm_x86_ops.set_irq(vcpu);
> > -               }
> > +       } else if (kvm_cpu_has_injectable_intr(vcpu) &&
> > +                  kvm_x86_ops.interrupt_injection_allowed(vcpu)) {
> > +               kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu), false);
> > +               kvm_x86_ops.set_irq(vcpu);
> >         }
> So, that's what this mess was all about! Well, this certainly looks better.

Right?  I can't count the number of times I've looked at this code and
wondered what the hell it was doing.

Side topic, I just realized you're reviewing my original series.  Paolo
commandeered it to extend it to SVM. https://patchwork.kernel.org/cover/11508679/