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

Alexander Graf
2010-04-23T14:24:59Z

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@amd.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