linux-kernel - Re: [RFC PATCH 01/11] Add basic support for gcc profiler instrumentation

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Message-ID: <477D34FB.6070601@goop.org>
Date:	Thu, 03 Jan 2008 11:18:19 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Chris Wright <chrisw@...hat.com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Gregory Haskins <ghaskins@...ell.com>,
	LKML <linux-kernel@...r.kernel.org>,
	virtualization@...ts.linux-foundation.org,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"William L. Irwin" <sparclinux@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 01/11] Add basic support for gcc profiler	instrumentation

Chris Wright wrote:
> * Steven Rostedt (rostedt@...dmis.org) wrote:
>   
>> Hmm, I know paravirt-ops had an issue with mcount in the RT tree. I can't
>> remember the exact issues, but it did have something to do with the way
>> parameters were passed in.
>>
>> Chris, do you remember what the issues were?
>>     
>
> Yes, paravirt ops have a well-specified calling convention (register
> based).  There was a cleanup that Andi did that caused the problem
> because it removed all the "fastcall" annotations since -mregparm=3
> is now always on for i386.  Since MCOUNT disables REGPARM the calling
> convention changes (caller pushes to stack, callee expects register)
> chaos ensues.  I sent a patch to fix that quite some months back, but
> it went stale and I neglected to update it.  Would you like me to dig
> it up refresh and resend?

Ingo/Andrew have been accepting patches to systematically remove all the
fastcall annotations from the kernel, so adding them back isn't going to
help.

Ingo and I discussed whether we need to reannotate paravirt.h (either
with fastcall or something else indicating a register-only calling
convention), specifically because of the -pg issue, but I think the
conclusion was that whatever problem existed no longer does, and there's
no incompatibility between -pg and regparm.

    J
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