Re: KVM handling external interrupts

[Jan Kiszka <jan.kiszka@web.de>]

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On 2012-06-07 12:34, Nadav Har'El wrote:
>>  - You discuss interrupt delivery without stating that you have MSIs in
>>    mind. Some aspects may be helpful for legacy interrupts as well, but
>>    you obviously can't achieve exit-less operation there. Not an issue,
>>    should just be made clear.
> 
> Can you eleborate on why exit-less operation cannot be achieved without
> MSI? Doesn't the VMCS flag to avoid exiting on external interrupts
> apply to any interrupts? Or something else won't work?

The guest needs to interact with the IOAPIC. And this resource is shared
between host and guest. It can't be passed through.

> 
> In any case, you're right that our implementation and tests all used
> MSI.
> 
>>> need paravirtualize the guest: no if you have x2APIC.
>>
>> ...and the guest makes use of it. This excludes older OSes. When did
>> Windows start to use it?
> 
> Iff you can't use x2APIC, and don't want to paravirtualize

Often, it is more about "... _can't_ paravirtualize". :)

> the guest, you still get exit-less interrupt *delivery*, which as we
> showed in the benchmarks, gets you more than half of the performance
> improvement (although with newer KVM's improvement in EOI emulation
> performance, the over-half improvement should be somewhat less pronounced).

Yes, I understood this, and I think looking at direct delivery would be
a good first step to check if this could eventually become an upstream
feature. It should even be beneficial for legacy interrupts.

Jan


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