linux-kernel - Re: [PATCH v1 5/5] KVM: Expose a version 1 architectural PMU to guests

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Message-ID: <20110517194117.GA26184@elte.hu>
Date:	Tue, 17 May 2011 21:41:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH v1 5/5] KVM: Expose a version 1 architectural PMU to
 guests

* Avi Kivity <avi@...hat.com> wrote:

> Caveats:
> - counters that have PMI (interrupt) enabled stop counting after the
>   interrupt is signalled.  This is because we need one-shot samples
>   that keep counting, which perf doesn't support yet

Hm, do you need more than perf_event::event_limit, or something special?

> - some combinations of INV and CMASK are not supported

Could you please describe this better, where does this limit come from?
If perf then this needs fixing.

> - counters keep on counting in the host as well as the guest

I suspect fixing this either needs a hw filter feature, or the ability to 
disable/enable these events across VM exits/entries.

I would imagine the disable/enable to be rather expensive so hw help would be 
preferred ...

I didnt see anything objectionable in your patches, but i'd like to have 
Peter's Acked-by as well before we go forward. I think that in the long run 
having a virtio-perf gateway would allow us a lot more tricks than just 
arch-perfmon emulation:

 - we could do things like propagate guest side traces over to the host

 - we could control from the host which events we measure on the guest side

 - etc.

How would you like to handle the flow of patches - should we merge #1,#2,#3 in 
perf/core and you'd then merge #4,#5 via the KVM tree once the first bits hit 
upstream?

We could also set up a separate branch for these three commits, which you could 
pull - this would allow all this to still hit .40.

Thanks,

	Ingo
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