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

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Date:	Fri, 23 Apr 2010 16:24:54 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Joerg Roedel <joerg.roedel@....com>
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 23.04.2010, at 16:22, Joerg Roedel wrote:

> 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.

So we modify the L2 rflags and then trigger a #vmexit, leaving the l2 state broken?

Alex

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