Re: [PATCH 2/2] KVM: x86: check_nested_events if there is an injectable NMI

[Paolo Bonzini]

On 23/04/20 17:35, Sean Christopherson wrote:
> On Thu, Apr 23, 2020 at 05:10:45PM +0200, Paolo Bonzini wrote:
>> On 23/04/20 16:42, Sean Christopherson wrote:
>>> On Tue, Apr 14, 2020 at 04:11:07PM -0400, Cathy Avery wrote:
>>>> With NMI intercept moved to check_nested_events there is a race
>>>> condition where vcpu->arch.nmi_pending is set late causing
>>> How is nmi_pending set late?  The KVM_{G,S}ET_VCPU_EVENTS paths can't set
>>> it because the current KVM_RUN thread holds the mutex, and the only other
>>> call to process_nmi() is in the request path of vcpu_enter_guest, which has
>>> already executed.
>>>
>> I think the actual cause is priority inversion between NMI and
>> interrupts, because NMI is added last in patch 1.
> Ah, that makes more sense.  I stared/glared at this exact code for a long
> while and came to the conclusion that the "late" behavior was exclusive to
> interrupts, would have been a shame if all that glaring was for naught.
> 

Ah no, it's a bug in Cathy's patch and it's a weird one.

The problem is that on AMD you exit guest mode with the NMI latched and
GIF=0.  So check_nested_events should enable the NMI window in addition
to causing a vmexit.

So why does it work?  Because on AMD we don't have (yet)
nested_run_pending, so we just check if we already have a vmexit
scheduled and if so return -EBUSY.  The second call causes
inject_pending_event to return -EBUSY and thus go through KVM_REQ_EVENT
again, which enables the NMI window.

Paolo