linux-kernel - Re: [PATCH] KVM: SVM: Don't intercept IRET when injecting NMI and vNMI is enabled

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Message-ID: <ZSWO6x-mFg37uIpq@google.com>
Date:   Tue, 10 Oct 2023 10:50:35 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Santosh Shukla <santosh.shukla@....com>
Subject: Re: [PATCH] KVM: SVM: Don't intercept IRET when injecting NMI and
 vNMI is enabled

On Tue, Oct 10, 2023, Maxim Levitsky wrote:
> У вт, 2023-10-10 у 07:46 -0700, Sean Christopherson пише:
> > On Tue, Oct 10, 2023, Maxim Levitsky wrote:
> > > У пн, 2023-10-09 у 14:29 -0700, Sean Christopherson пише:
> > > > Note, per the APM, hardware sets the BLOCKING flag when software directly
> > > > directly injects an NMI:
> > > > 
> > > >   If Event Injection is used to inject an NMI when NMI Virtualization is
> > > >   enabled, VMRUN sets V_NMI_MASK in the guest state.
> > > 
> > > I think that this comment is not needed in the commit message. It describes
> > > a different unrelated concern and can be put somewhere in the code but
> > > not in the commit message.
> > 
> > I strongly disagree, this blurb in the APM directly affects the patch.  If hardware
> > didn't set V_NMI_MASK, then the patch would need to be at least this:
> 
> I don't see how 'the blurb in the APM' relates to the removal of the 
> IRET intercept, which is what this patch is about.

No, it's not *just* about IRET interception.  This patch also guards:

	svm->nmi_masked = true;

If the reader doesn't already know that hardware sets V_NMI_BLOCK_MASK on direct
injection, as was the case for me when I stumbled upon this issue, it's not at
all obvious that not doing something analogous to setting nmi_masked is correct.

I mentioned only IRET interception in the shortlog because that's the only practical
impact of the change.  I can massage the shortlog if it's confusing/misleading,
but I really don't want to drop the reference to hardware setting V_NMI_BLOCK_MASK.