Re: [PATCH] KVM: VMX: Fix race between pending IRQ and NMI

[Avi Kivity <avi@redhat.com>]

Avi Kivity wrote:
> Jan Kiszka wrote:
>> Jiajun kindly provided me a RHEL kernel and initrd (2.6.18-53-el5) which
>> I ran for a while (or booted a few times) to trigger the hang. Basically
>> you need high IRQ load (preferably via LAPIC, to exploit that un-acked
>> IRQs will block low-prio IRQs as well) + high NMI load (e.g. via NMI
>> watchdog).
>>   
>
> I was able to reproduce it easily by zapping the mmu every second.
>
> Attached is a patch the fixes it for me.  Basically it avoids the nmi 
> path if an interrupt is being injected.  This is closer to my event 
> queue plan, and also is similar to what the code does today with 
> exceptions (avoid ->inject_pending_irq() if an exception is pending).
>

Oh, and I think this is more correct than the previous approach of 
letting the nmi preempt the interrupt.

The nmi handler could change the tpr to mask the preempted interrupt; 
but the code would not notice that.  Once the interrupt was injected the 
guest would see an interrupt at a higher priority than it has programmed 
the hardware to allow.

Basically, once we commit to an interrupt via kvm_cpu_get_interrupt(), 
we must inject it before the any instruction gets executed.

I don't think any real guest would notice, though.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.