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

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Message-ID: <1ca607bcb4931b7f5e14e6c064264d86e58fd3ce.camel@redhat.com>
Date:   Tue, 10 Oct 2023 19:06:25 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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

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.

If the hardware was not to set the V_NMI_BLOCKING_MASK during EVENTINJ NMI injection, 
we would have had a bigger problem, a problem which would have to be addressed 
before this patch, because kvm reads back the V_NMI_BLOCKING_MASK 
(see: svm_get_nmi_mask()) to check if NMI is blocked, something that
has no relation to the IRET interception.


Best regards,
	Maxim Levtsky


> 
> --
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index b7472ad183b9..d34ee3b8293e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3569,8 +3569,12 @@ static void svm_inject_nmi(struct kvm_vcpu *vcpu)
>  	if (svm->nmi_l1_to_l2)
>  		return;
>  
> -	svm->nmi_masked = true;
> -	svm_set_iret_intercept(svm);
> +	if (is_vnmi_enabled(svm)) {
> +		svm->vmcb->control.int_ctl |= V_NMI_BLOCKING_MASK;
> +	} else {
> +		svm->nmi_masked = true;
> +		svm_set_iret_intercept(svm);
> +	}
>  	++vcpu->stat.nmi_injections;
>  }
>  
> 
> base-commit: 86701e115030e020a052216baa942e8547e0b487