Re: [PATCH V3] perf & kvm: Enhance perf to collect KVM guest os statistics from host side

Avi Kivity
2010-04-14T10:20:21Z

On 04/14/2010 01:14 PM, Sheng Yang wrote:
>
>> I wouldn't like to depend on model specific behaviour.
>>
>> One option is to read all the information synchronously and store it in
>> a per-cpu area with atomic instructions, then queue the NMI. Another
>> option is to have another callback which tells us that the NMI is done,
>> and have a busy loop wait until the NMI is delivered.
>>
>>
> Callback seems too heavy, may affect the performance badly. Maybe a short
> queue would help, though this one is more complex.
>

The patch we're replying to adds callbacks (to read rip, etc.), so it's
no big deal. For the queue solution, a queue of size one would probably
be sufficient even if not guaranteed by the spec. I don't see how the
cpu can do another guest entry without delivering the NMI.

> But I am still curious if we extend the region, how much it would help. Would
> get a result soon...
>

Yes, interesting to see what the latency is. If it's reasonably short
(and I expect it will be so), we can do the busy wait solution.

If we have an NMI counter somewhere, we can simply wait until it changes.

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