linux-kernel - Re: [patch] x86, mm: pass in 'total' to __copy_from_user

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Date:	Sat, 28 Feb 2009 17:40:51 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	David Miller <davem@...emloft.net>,
	torvalds@...ux-foundation.org, mingo@...e.hu,
	nickpiggin@...oo.com.au, sqazi@...gle.com,
	linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()

Arjan van de Ven wrote:
> 
> the reason that movntq and co are faster is because you avoid the
> write-allocate behavior of the caches....
> 
> the cache polluting part of it I find hard to buy for general use (as
> this discussion shows)... that will be extremely hard to measure as
> a real huge thing, while the WA part is like a 1.5x to 2x thing.
> 

Note that hardware *can* (which is not the same thing as hardware
*will*) elide the write-allocate behavior.  We did that at Transmeta for
rep movs and certain other instructions which provably filled in entire
cache lines.  I haven't investigated if newer Intel CPUs do that in the
"fast rep movs" case.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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