Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()

From: "H. Peter Anvin" <hpa@zytor.com>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Andi Kleen <andi@firstfloor.org>,
	David Miller <davem@davemloft.net>,
	torvalds@linux-foundation.org, mingo@elte.hu,
	nickpiggin@yahoo.com.au, sqazi@google.com,
	linux-kernel@vger.kernel.org, tglx@linutronix.de
Subject: Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()
Date: Sat, 28 Feb 2009 17:40:51 -0800	[thread overview]
Message-ID: <49A9E7A3.4050102@zytor.com> (raw)
In-Reply-To: <20090228173813.6d86c0ef@infradead.org>

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.