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

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Message-ID: <4B7C7023.7060602@zytor.com>
Date:	Wed, 17 Feb 2010 14:39:31 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Luca Barbieri <luca@...a-barbieri.com>
CC:	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

On 02/17/2010 03:42 AM, Luca Barbieri wrote:
> This patch uses SSE movlps to perform 64-bit atomic reads and writes.
> 
> According to Intel manuals, all aligned 64-bit reads and writes are
> atomically, which should include movlps.
> 
> To do this, we need to disable preempt, clts if TS was set, and
> restore TS.
> 
> If we don't need to change TS, using SSE is much faster.
> 
> Otherwise, it should be essentially even, with the fastest method
> depending on the specific architecture.
> 
> Another important point is that with SSE atomic64_read can keep the
> cacheline in shared state.
> 
> If we could keep TS off and reenable it when returning to userspace,
> this would be even faster, but this is left for a later patch.
> 
> We use SSE because we can just save the low part %xmm0, whereas using
> the FPU or MMX requires at least saving the environment, and seems
> impossible to do fast.
> 
> Signed-off-by: Luca Barbieri <luca@...a-barbieri.com>

I'm a bit unhappy about this patch.  It seems to violate the assumption
that we only ever use the FPU state guarded by
kernel_fpu_begin()..kernel_fpu_end() and instead it uses a local hack,
which seems like a recipe for all kinds of very subtle problems down the
line.

Unless the performance advantage is provably very compelling, I'm
inclined to say that this is not worth it.

	-hpa
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