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

[Gleb Natapov <gleb@redhat.com>]

On Wed, Apr 21, 2010 at 05:14:04PM +0200, Jan Kiszka wrote:
> > No you don't. I was told that software should be prepared to handle NMI
> > after MOV SS. What part of SDM does this contradict? I found nothing in
> > latest SDM.
> 
> [ updated to March 2010 version ]
> 
> To sum up the scenario again, I think it started with
> 
> • If the “NMI-window exiting” VM-execution control is 1, a VM exit occurs before
>   execution of any instruction if there is no virtual-NMI blocking and there is no
>   blocking of events by MOV SS (see Table 21-3). (A logical processor may also
>   prevent such a VM exit if there is blocking of events by STI.) Such a VM exit
>   occurs immediately after VM entry if the above conditions are true (see Section
>   23.6.6).
> 
> 
> We included STI into the NMI shadow, but we /may/ get early exits on
> some processors according to the statement above. According to your
> latest info, we can also get that when the MOV SS shadow is on!? But
> simply allowing NMI injection under MOV SS is not possible:
> 
> 23.3 CHECKING AND LOADING GUEST STATE
> 23.3.1.5 Checks on Guest Non-Register State
> 
> • Interruptibility state.
>   ...
>   — Bit 1 (blocking by MOV-SS) must be 0 if the valid bit (bit 31) in the VM-entry
>     interruption-information field is 1 and the interruption type (bits 10:8) in that
>     field has value 2, indicating non-maskable interrupt (NMI).
> 
> 
> And doing this for STI sounds risky too:
> 
>   — A processor may require bit 0 (blocking by STI) to be 0 if the valid bit (bit 31)
>     in the VM-entry interruption-information field is 1 and the interruption type
>     (bits 10:8) in that field has value 2, indicating NMI. Other processors may not
>     make this requirement.
> 
> 
> Should we start stepping over the shadow like we do for svm?
> 
Intel's answer is that text above describes model-specific behaviour
in bare metal which can block NMI for one instruction after STI/MOV SS,
but since software should not rely on model-specific behaviour we can
safely inject NMI after STI/MOV SS (clearing "blocked by STI/MOV SS" bit
before injecting).

--
			Gleb.