linux-kernel - Re: copy_{to,from}

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Date:	Sun, 11 Jan 2009 03:09:23 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Brad Parker <brad@...ltoe.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: copy_{to,from}_user


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2009-01-09 at 12:52 -0500, Brad Parker wrote:
> > I have a question about copy_{to,from}_user.  
> > 
> > Most implementations I've seen do in-order copies and notice when an
> > exception occurs and report back the progress.  This is straight
> > forward.
> > 
> > (but to be honest, I have suspicions about how just how accurate those
> > reports are i.e. +/- 1-3 bytes on some architectures)
> > 
> > On some cpu's it is advantageous to do an out-of-order copy to take
> > advantage of various cache fill mechanisms.
> > 
> > The problem is that the out-of-order copy makes it impossible to know
> > where the exception occurred (in terms of progress).
> > 
> > Would it be permissible to have a version of copy_{to,from}_user which
> > does an out-of-order copy and when an exception occurs, restarts the
> > copy from the beginning using a simple in-order copy, to make it
> > possible to identify where the exception occurs?
> > 
> > The idea is that exceptions are rare and so the performance hit of doing
> > the "recopy" would be minimal and would provide the required accuracy.
> 
> x86_64 already does some unrolling and is inaccurate as to where exactly
> it happens. The only thing that is very important is that you _never_
> say you copied more than you actually did.
> 
> That was the source of a data corruption bug a while ago, the code did 
> something like sequences: read 8 words, write 8 words. And reported the 
> number of bytes read, instead of bytes written, which is an 
> over-estimation.

you sure must have meant 'write 7 words' or something like that?

	Ingo
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