linux-kernel - Re: [patches] [PATCH 2.6.21 review I] [21/25] x86_64: a memcpy that tries to reduce cache pressure

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Message-Id: <45D04806.76E4.0078.0@novell.com>
Date:	Mon, 12 Feb 2007 09:57:10 +0000
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Bryan O'Sullivan" <bos@...hscale.com>, "Andi Kleen" <ak@...e.de>
Cc:	"Roland Dreier" <rolandd@...co.com>,
	<linux-kernel@...r.kernel.org>, <patches@...-64.org>
Subject: Re: [patches] [PATCH 2.6.21 review I] [21/25] x86_64: a memcpy
	that tries to reduce cache pressure

>>> Andi Kleen <ak@...e.de> 10.02.07 12:50 >>>
>
>From: "Bryan O'Sullivan" <bos@...hscale.com>
>
>This copy routine is memcpy-compatible, but on some architectures will use
>cache-bypassing loads to avoid bringing the source data into the cache.
>
>One case where this is useful is when a device issues a DMA to a memory
>region, and the CPU must copy the DMAed data elsewhere before doing any work
>with it.  Since the source data is read-once, write-never from the CPU's
>perspective, caching the data at those addresses can only evict potentially
>useful data.
>
>We provide an x86_64 implementation that uses SSE non-temporal loads, and a
>generic version that falls back to plain memcpy.
>
>Implementors for other arches should not use cache-bypassing stores to the
>destination, as in most cases, the destination is accessed almost immediately
>after a copy finishes.

This looks a little strange to me:
- the first 128 bytes are still going through the cache
- up to 192 bytes past the copied area are being marked non-temporal, while
  there's nothing known about that area
- sfence seems questionable here, I would have thought this should be lfence,
  or perhaps even none at all

Minor remarks would be to remove the double .align before .L12 and replace
or-ing a register with itself by test.

Jan
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