Re: [PATCH] KVM: VMX: Translate interrupt shadow when waiting on NMI window

[Jan Kiszka <jan.kiszka@siemens.com>]

Gleb Natapov wrote:
> On Wed, Apr 21, 2010 at 04:17:03PM +0200, Jan Kiszka wrote:
>> Gleb Natapov wrote:
>>> On Tue, Feb 16, 2010 at 11:37:15AM +0100, Jan Kiszka wrote:
>>>> Gleb Natapov wrote:
>>>>> On Tue, Feb 16, 2010 at 11:27:07AM +0100, Jan Kiszka wrote:
>>>>>> Gleb Natapov wrote:
>>>>>>> On Tue, Feb 16, 2010 at 11:14:45AM +0100, Jan Kiszka wrote:
>>>>>>>> Gleb Natapov wrote:
>>>>>>>>> On Tue, Feb 16, 2010 at 11:04:10AM +0100, Jan Kiszka wrote:
>>>>>>>>>> Gleb Natapov wrote:
>>>>>>>>>>> On Tue, Feb 16, 2010 at 10:16:12AM +0100, Jan Kiszka wrote:
>>>>>>>>>>>> Found while browsing Xen code: While we assume that the STI interrupt
>>>>>>>>>>>> shadow also inplies virtual NMI blocking, some processors may have a
>>>>>>>>>>>> different opinion (SDM 3: 22.3). To avoid misunderstandings that would
>>>>>>>>>>>> cause endless VM entry attempts, translate STI into MOV SS blocking when
>>>>>>>>>>>> requesting the NMI window.
>>>>>>>>>>>>
>>>>>>>>>>> Why not just remove "block by STI" check in vmx_nmi_allowed()? IIRC this
>>>>>>>>>>> is documented that on some CPUs STI does not block NMI.
>>>>>>>>>>>
>>>>>>>>>> Probably because we will stumble and fall on those CPUs that do care.
>>>>>>>>>>
>>>>>>>>> But this defines behaviour of cpu _we_ emulate. So on _our_ cpu NMI will
>>>>>>>>> not be blocked by STI.
>>>>>>>> The host CPU decides if it accepts an NMI injections while
>>>>>>> Are you sure? I haven't found such check during VMENTRY.
>>>>>> I also only find the explicitly stated exclusion of MOV SS blocking vs.
>>>>>> NMI injection. If we can rely on this, removing STI blocking from
>>>>>> vmx_nmi_allowed should suffice. Or, better, can we get an official
>>>>>> confirmation from Intel?
>>>>>>
>>>>> SDM 2b says about STI instruction:
>>>>> The IF flag and the STI and CLI instructions do not prohibit the
>>>>> generation of exceptions and NMI interrupts. NMI interrupts (and SMIs)
>>>>> may be blocked for one macroinstruction following an STI.
>>>> Yes, it's likely that this is the architectural reason for the delayed
>>>> NMI window signaling after STI. Still, we are looking for the
>>>> entry-check logic.
>>>>
>>> Will ask Intel.
>>>
>> Just remembered that there was some open topic... Did your ask? Any answer?
>>
> I did and got answer last week :) The answer is that NMI is blocked only
> if GUEST_INTR_STATE_NMI flag is set. MOV SS and STI shouldn't block NMI,
> so vmx_nmi_allowed() should check only GUEST_INTR_STATE_NMI flag.

Cool, that's now increasing my level of confusion again: :(

Thought we only wanted to confirm that it's still safe to inject NMIs
when blocked-by-STI is set. Now we hear that it's also safe when MOV SS
is active? That would directly contradict the SDM (at least the version
I have at hand: June 2009). Or did I misunderstand the answer?

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux