linux-kernel - Re: [PATCH v1 0/5] KVM in-guest performance monitoring

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Message-ID: <20110512142449.GI8707@8bytes.org>
Date:	Thu, 12 May 2011 16:24:49 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	Jan Kiszka <jan.kiszka@...mens.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH v1 0/5] KVM in-guest performance monitoring

On Thu, May 12, 2011 at 04:31:38PM +0300, Avi Kivity wrote:
> - when the cpu gains support for virtualizing the architectural feature,  
> we transparently speed the guest up, including support for live  
> migrating from a deployment that emulates the feature to a deployment  
> that properly virtualizes the feature, and back.  Usually the  
> virtualized support will beat the pants off any paravirtualization we can 
> do
> - following an existing spec is a lot easier to get right than doing  
> something from scratch
> - no need to meticulously document the feature

Need to be done, but not problematic I think.

> - easier testing

Testing shouldn't be different on both variants I think.

> - existing guest support - only need to write the host side (sometimes  
> the only one available to us)

Otherwise I agree.

> Paravirtualizing does have its advantages.  For the PMU, for example, we  
> can have a single hypercall read and reprogram all counters, saving  
> *many* exits.  But I think we need to start from the architectural PMU  
> and see exactly what the problems are, before we optimize it to death.

The problem certainly is that with arch-pmu we add a lot of msr-exits to
the guest-context-switch path if it uses per-task profiling. Depending
on the workload this can very much distort the results.

	Joerg

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