Re: [PATCH 20/24] Correct handling of interrupt injection

[Avi Kivity <avi@redhat.com>]

On 06/14/2010 03:29 PM, Avi Kivity wrote:
> On 06/13/2010 03:32 PM, Nadav Har'El wrote:
>> When KVM wants to inject an interrupt, the guest should think a real 
>> interrupt
>> has happened. Normally (in the non-nested case) this means checking 
>> that the
>> guest doesn't block interrupts (and if it does, inject when it 
>> doesn't - using
>> the "interrupt window" VMX mechanism), and setting up the appropriate 
>> VMCS
>> fields for the guest to receive the interrupt.
>>
>> However, when we are running a nested guest (L2) and its hypervisor (L1)
>> requested exits on interrupts (as most hypervisors do), the most 
>> efficient
>> thing to do is to exit L2, telling L1 that the exit was caused by an
>> interrupt, the one we were injecting; Only when L1 asked not to be 
>> notified
>> of interrupts, we should to inject it directly to the running guest 
>> L2 (i.e.,
>> the normal code path).
>>
>> However, properly doing what is described above requires invasive 
>> changes to
>> the flow of the existing code, which we elected not to do in this stage.
>> Instead we do something more simplistic and less efficient: we modify
>> vmx_interrupt_allowed(), which kvm calls to see if it can inject the 
>> interrupt
>> now, to exit from L2 to L1 before continuing the normal code. The 
>> normal kvm
>> code then notices that L1 is blocking interrupts, and sets the interrupt
>> window to inject the interrupt later to L1. Shortly after, L1 gets the
>> interrupt while it is itself running, not as an exit from L2. The 
>> cost is an
>> extra L1 exit (the interrupt window).
>>
>
> That's a little sad.

It can also be broken if the guest chooses to keep interrupts disabled 
during exits, and instead ask vmx to ack interrupts.  The guest can then 
vmread the vector number and dispatch the interrupt itself.

-- 
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