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

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Date:	Sat, 28 Feb 2009 09:53:25 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Nick Piggin <nickpiggin@...oo.com.au>,
	Salman Qazi <sqazi@...gle.com>, davem@...emloft.net,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>
Subject: Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()

On Sat, 28 Feb 2009 09:42:18 -0800 (PST)
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> That's almost unacceptable in the long run. With a 8MB L3 cache - and
> a compile sequence, do we really want to go out to memory to write
> the .S file, and then have the assembler go out to memory to read it
> back? For a compile, that _probably_ is all fine (the compiler in
> particular will have enough data structures around that it's not
> going to fit in the cache anyway), but I'm seeing leaner compilers
> and other cases where forcing things out all the way on the bus is
> simply the wrong thing.

non-temporal is almost always wrong

one of the valid cases is where you KNOW you'll be doing dma for all of
the memory very shortly (say, an O_SYNC/O_DIRECT write of 4Kb) since
that would tend to evict it from cache anyway.

but beyond that... not so much.

-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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