linux-kernel - Re: [GIT PULL] perf changes for v3.8

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Message-ID: <CA+55aFwSYyoLmdDJ5xsdwcWLThYQ7oieoeWDV2g+wV3CJk4vtg@mail.gmail.com>
Date:	Thu, 13 Dec 2012 09:02:50 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] perf changes for v3.8

Btw, I do *not* think that you should necessariyl default to 'H' for
host-only mode.

The way it should work is that ":pp", ":ppH" and ":ppV" are all different.

 - "cycles:ppH" means: I want precise cycles only for the host case

 - "cycles:ppV" means: I want precise cycles, and I want the VM too

   This would result in EOPNOTSUPP for the case we know is buggy (but
presumably work on some other CPUs that don't have the problem)

 - "cycles:pp" is "I want precise cycles, and I don't care about virtualization"

   This would do whatever works. So it would basically become
host-only, but if you don't want precise cycles (so no ":pp") then
whatever our old behavior was (presumably "profile the virtual machine
too") would be what happens.

That sounds like (a) the interface that people want and (b) entirely
backwards-compatible for all cases that can matter (where "oops, I
crashed the VM" case does not matter).

                Linus
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