linux-kernel - Re: [PATCH 2/2] KVM: x86: check_nested_events if there is an injectable NMI

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Date:   Thu, 23 Apr 2020 08:35:31 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Cathy Avery <cavery@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, vkuznets@...hat.com, wei.huang2@....com
Subject: Re: [PATCH 2/2] KVM: x86: check_nested_events if there is an
 injectable NMI

On Thu, Apr 23, 2020 at 05:10:45PM +0200, Paolo Bonzini wrote:
> On 23/04/20 16:42, Sean Christopherson wrote:
> > On Tue, Apr 14, 2020 at 04:11:07PM -0400, Cathy Avery wrote:
> >> With NMI intercept moved to check_nested_events there is a race
> >> condition where vcpu->arch.nmi_pending is set late causing
> > How is nmi_pending set late?  The KVM_{G,S}ET_VCPU_EVENTS paths can't set
> > it because the current KVM_RUN thread holds the mutex, and the only other
> > call to process_nmi() is in the request path of vcpu_enter_guest, which has
> > already executed.
> > 
> 
> I think the actual cause is priority inversion between NMI and
> interrupts, because NMI is added last in patch 1.

Ah, that makes more sense.  I stared/glared at this exact code for a long
while and came to the conclusion that the "late" behavior was exclusive to
interrupts, would have been a shame if all that glaring was for naught.