linux-kernel - Re: [PATCH 1/8] KVM: SVM: Fix nested nmi handling

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Message-ID: <20100423142258.GG31537@amd.com>
Date:	Fri, 23 Apr 2010 16:22:58 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Alexander Graf <agraf@...e.de>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/8] KVM: SVM: Fix nested nmi handling

On Fri, Apr 23, 2010 at 04:19:40PM +0200, Alexander Graf wrote:
> 
> On 23.04.2010, at 16:13, Joerg Roedel wrote:
> 
> > On Fri, Apr 23, 2010 at 03:46:07PM +0200, Alexander Graf wrote:
> >> 
> >> On 22.04.2010, at 12:33, Joerg Roedel wrote:
> >> 
> >>> The patch introducing nested nmi handling had a bug. The
> >>> check does not belong to enable_nmi_window but must be in
> >>> nmi_allowed. This patch fixes this.
> >>> 
> >>> Signed-off-by: Joerg Roedel <joerg.roedel@....com>
> >>> ---
> >>> arch/x86/kvm/svm.c |   16 +++++++++-------
> >>> 1 files changed, 9 insertions(+), 7 deletions(-)
> >>> 
> >>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> >>> index ab78eb8..ec20584 100644
> >>> --- a/arch/x86/kvm/svm.c
> >>> +++ b/arch/x86/kvm/svm.c
> >>> @@ -2771,8 +2771,12 @@ static int svm_nmi_allowed(struct kvm_vcpu *vcpu)
> >>> {
> >>> 	struct vcpu_svm *svm = to_svm(vcpu);
> >>> 	struct vmcb *vmcb = svm->vmcb;
> >>> -	return !(vmcb->control.int_state & SVM_INTERRUPT_SHADOW_MASK) &&
> >>> -		!(svm->vcpu.arch.hflags & HF_NMI_MASK);
> >>> +	int ret;
> >>> +	ret = !(vmcb->control.int_state & SVM_INTERRUPT_SHADOW_MASK) &&
> >>> +	      !(svm->vcpu.arch.hflags & HF_NMI_MASK);
> >>> +	ret = ret && gif_set(svm) && nested_svm_nmi(svm);
> >>> +
> >>> +	return ret;
> >>> }
> >>> 
> >>> static bool svm_get_nmi_mask(struct kvm_vcpu *vcpu)
> >>> @@ -2841,11 +2845,9 @@ static void enable_nmi_window(struct kvm_vcpu *vcpu)
> >>> 	 * Something prevents NMI from been injected. Single step over possible
> >>> 	 * problem (IRET or exception injection or interrupt shadow)
> >>> 	 */
> >>> -	if (gif_set(svm) && nested_svm_nmi(svm)) {
> >>> -		svm->nmi_singlestep = true;
> >>> -		svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
> >>> -		update_db_intercept(vcpu);
> >>> -	}
> >>> +	svm->nmi_singlestep = true;
> >>> +	svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
> >>> +	update_db_intercept(vcpu);
> >> 
> >> So we're always messing with the nested guest state when the host
> >> wants to inject an nmi into the l1 guest? Is that safe?
> > 
> > Why not? We can't inject an NMI directly into L2 if the nested
> > hypervisor intercepts it.
> 
> So where did the code go that does the #vmexit in case the nested
> hypervisor does intercept it? It used to be nested_svm_nmi(), right?

No, nested_svm_nmi runs in atomic context where we can't emulate a
vmexit. We set exit_required and emulate the vmexit later.

	Joerg


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