Re: [PATCH 13/15] Add NMI injection support to SVM.

[Jan Kiszka <jan.kiszka@web.de>]

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Gleb Natapov wrote:
> On Sun, Apr 19, 2009 at 03:27:22PM +0200, Jan Kiszka wrote:
>> Gleb Natapov wrote:
>>> On Fri, Apr 17, 2009 at 09:55:45PM +0200, Jan Kiszka wrote:
>>>> Gleb Natapov wrote:
>>>>> Signed-off-by: Gleb Natapov <gleb@redhat.com>
>>>>> ---
>>>>>  arch/x86/include/asm/kvm_host.h |    1 +
>>>>>  arch/x86/kvm/svm.c              |   49 +++++++++++++++++++++++++++++++++++++-
>>>>>  2 files changed, 48 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>>>> index 8b6f6e9..057a612 100644
>>>>> --- a/arch/x86/include/asm/kvm_host.h
>>>>> +++ b/arch/x86/include/asm/kvm_host.h
>>>>> @@ -766,6 +766,7 @@ enum {
>>>>>  #define HF_GIF_MASK		(1 << 0)
>>>>>  #define HF_HIF_MASK		(1 << 1)
>>>>>  #define HF_VINTR_MASK		(1 << 2)
>>>>> +#define HF_NMI_MASK		(1 << 3)
>>>>>  
>>>>>  /*
>>>>>   * Hardware virtualization extension instructions may fault if a
>>>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>>>>> index c605477..cd60fd7 100644
>>>>> --- a/arch/x86/kvm/svm.c
>>>>> +++ b/arch/x86/kvm/svm.c
>>>>> @@ -1834,6 +1834,13 @@ static int cpuid_interception(struct vcpu_svm *svm, struct kvm_run *kvm_run)
>>>>>  	return 1;
>>>>>  }
>>>>>  
>>>>> +static int iret_interception(struct vcpu_svm *svm, struct kvm_run *kvm_run)
>>>>> +{
>>>>> +	svm->vmcb->control.intercept &= ~(1UL << INTERCEPT_IRET);
>>>>> +	svm->vcpu.arch.hflags &= ~HF_NMI_MASK;
>>>>> +	return 0;
>>>>> +}
>>>> First, this must return 1 (or set an exit reason, but there is no reason
>>>> to escape to user space here). And second, I think a corner case is not
>>>> handled the same way as on real iron: If there is already the next NMI
>>>> waiting, we will inject it before iret, not after its execution as it
>>>> should be.
>>>>
>>>> No easy solution for this yet. Maybe emulating iret, but there is no
>>>> implementation, specifically for protected mode. Maybe setting a
>>>> breakpoint. Or maybe enforcing a single step exception. Nothing trivial
>>>> in this list. On the other hand, this may only be a slight imprecision
>>>> of the virtualization. Need to think about it.
>>>>
>>> What about this:
>>> Instead of clearing HF_NMI_MASK in iret_interception() we can set
>>> another flag (HF_IRET) and on guest entry clear HF_NMI_MASK (and
>>> HF_IRET) if HF_IRET is set, but do that after checking for NMI
>>> injection. The pending NMI will be injected on the next entry.
>>> Also not how real HW works, but may be better then current situation.
>> It's OK as a first step towards correct NMI emulation. Additionally, you
>> could enable the IRQ window interception in case the is an NMI pending.
>> The resulting behavior should then much like the VNMI mask emulation for
>> vmx.
>>
> Yeah, but the question is if IRQ windows is already opened will exit
> happens before or after IRET.
> 

Hey, this is band-aid, it won't heal all cases. ;)

Jan


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