linux-kernel - Re: [PATCH][RFC 2/23]: SCST core

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Date:	Tue, 16 Dec 2008 22:49:26 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Vladislav Bolkhovitin <vst@...b.net>,
	Steven Rostedt <srostedt@...hat.com>,
	Sam Ravnborg <sam@...nborg.org>, linux-scsi@...r.kernel.org,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Mike Christie <michaelc@...wisc.edu>,
	Jeff Garzik <jeff@...zik.org>,
	Boaz Harrosh <bharrosh@...asas.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, scst-devel@...ts.sourceforge.net,
	Bart Van Assche <bart.vanassche@...il.com>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Subject: Re: [PATCH][RFC 2/23]: SCST core


* Frédéric Weisbecker <fweisbec@...il.com> wrote:

> > All the above functionality is almost what we need. The only thing 
> > left, which I forgot to mention, is possibility to log also functions 
> > return value on exit. This is what TRACE_EXIT_RES() in SCST does. Is 
> > it possible to add those?
> 
> I want to add that on the function graph tracer. That can be done pretty 
> easily. The only problem comes with the type of the return value. Would 
> this tracer be supposed to always return a 64 bits value regardless of 
> the real typ of the value? There would be some pointless bytes on most 
> return values. I don't know how to proceed for this problem.

Things like mov ...,%eax are zero-extend so they'll zap the high 32 bits.

The real problem are byte return values generated via things like:

   movb $1, %al

those wont zero-extend, so you could get garbage in the output. One 
approach would be to try a quick hack just to see how common a problem 
this is.

We could extract the return type from the debuginfo, hash it in a 
read-mostly table and then look it up, but that seems complex both in 
terms of build overhead and in terms of runtime overhead.

	Ingo
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