linux-kernel - Re: [PATCH 09/10] x86-32: use SSE for atomic64

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Date:	Thu, 18 Feb 2010 19:14:30 +0100
From:	Luca Barbieri <luca@...a-barbieri.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>, mingo@...e.hu,
	a.p.zijlstra@...llo.nl, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/10] x86-32: use SSE for atomic64_read/set if available

> This is what kernel_fpu_begin/kernel_fpu_end is all about.  We
> definitely cannot leave TS cleared without the user space CPU state
> moved to its home location, or we have yet another complicated state to
> worry about.

It should be relatively simple to handle, since the current code
doesn't really rely on the TS flag but uses TS_USEDFPU.
It would mostly be a matter of making sure TS is restored on return to
userspace if necessary.

> I really feel that without a *strong* use case for this, there is
> absolutely no point.
For the specific 32-bit atomic64_t case, it is an improvement, but not
necessarily significant in the big picture.
Being able to efficiently use SSE in the kernel might however be more
broadly useful.
memcpy/memset/etc. (assuming SSE is the best option for these at least
on some processors) and checksums come to mind.
Also non-temporal SSE moves might be useful for things like memory
compaction without clobbering caches.
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