linux-kernel - RFC: paravirtualizing perf

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Message-ID: <526DBD7F.1010807@gmail.com>
Date:	Sun, 27 Oct 2013 19:27:27 -0600
From:	David Ahern <dsahern@...il.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Gleb Natapov <gleb@...hat.com>
CC:	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: RFC: paravirtualizing perf_clock

Often when debugging performance problems in a virtualized environment 
you need to correlate what is happening in the guest with what is 
happening in the host. To correlate events you need a common time basis 
(or the ability to directly correlate the two).

The attached patch paravirtualizes perf_clock, pulling the timestamps in 
VMs from the host using an MSR read if the option is available (exposed 
via KVM feature flag). I realize this is not the correct end code but it 
illustrates what I would like to see -- host and guests using the same 
perf_clock so timestamps directly correlate.

Any suggestions on how to do this and without impacting performance. I 
noticed the MSR path seems to take about twice as long as the current 
implementation (which I believe results in rdtsc in the VM for x86 with 
stable TSC).

David

View attachment "0001-perf-kvm-x86-Paravirtualize-perf_clock-for-x86_64.patch" of type "text/plain" (4240 bytes)