linux-kernel - Re: [PATCH] arch/x86: use kernel_fpu_[begin|end] for RAID5 checksumming

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Date:	Wed, 02 May 2012 23:43:33 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	NeilBrown <neilb@...e.de>
CC:	Jim Kukunas <james.t.kukunas@...ux.intel.com>,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch/x86: use kernel_fpu_[begin|end] for RAID5 checksumming

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On 05/02/2012 09:57 PM, NeilBrown wrote:
> On Mon, 30 Apr 2012 16:51:15 -0700 Jim Kukunas 
> <james.t.kukunas@...ux.intel.com> wrote:
> 
>> Currently, the SSE and AVX xor functions manually save and 
>> restore the [X|Y]MM registers. Instead, we should use 
>> kernel_fpu_[begin|end].
>> 
>> This patch sacrifices some throughput,~5-10% for AVX and ~2% for 
>> SSE, in exchange for safety against future FPU corruption bugs.
>> 
>> Patch applies to md/for-next.
>> 
>> Signed-off-by: Jim Kukunas <james.t.kukunas@...ux.intel.com>
> 
> Hi Jim, I'm really not able to assess the validity of this patch.
> I don't doubt it exactly, but I'd value a second opinion just to
> be confident. Peter: can you review and comment?
> 
> Thanks, NeilBrown
> 

Are these functions ever called with unaligned arguments?

Other than that, the patch is correct, and is the preferred way to do
this, but I'm unhappy about the cost.  I have suggested to Jim a few
ways by which we might be able to mitigate the cost or even improve
performance, but the implementation depends very much on the above
question (unlike the RAID-6 code, which assumes aligned buffers at all
times.)

	-hpa
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