linux-kernel - Re: x86 memcpy performance

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Message-ID: <CAObL_7GNyj2u1smn+tmg1W+Su4uXL9OgYCT-k+9KPuSS4v0gXQ@mail.gmail.com>
Date:	Mon, 15 Aug 2011 14:35:26 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	melwyn lobo <linux.melwyn@...il.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	borislav.petkov@....com
Subject: Re: x86 memcpy performance

On Mon, Aug 15, 2011 at 2:26 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 08/15/2011 09:58 AM, Andrew Lutomirski wrote:
>> On Mon, Aug 15, 2011 at 12:12 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>> On 08/15/2011 08:36 AM, Andrew Lutomirski wrote:
>>>>
>>>> (*)  kernel_fpu_begin is a bad name.  It's only safe to use integer
>>>> instructions inside a kernel_fpu_begin section because MXCSR (and the
>>>> 387 equivalent) could contain garbage.
>>>>
>>>
>>> Uh... no, it just means you have to initialize the settings.  It's a
>>> perfectly good name, it's called kernel_fpu_begin, not kernel_fp_begin.
>>
>> I prefer get_xstate / put_xstate, but this could rapidly devolve into
>> bikeshedding. :)
>>
>
> a) Quite.
>
> b) xstate is not architecture-neutral.

Are there any architecture-neutral users of this thing?  If I were
writing generic code, I would expect:

kernel_fpu_begin();
foo *= 1.5;
kernel_fpu_end();

to work, but I would not expect:

kernel_fpu_begin();
use_xmm_registers();
kernel_fpu_end();

to make any sense.

Since the former does not actually work, I would hope that there is no
non-x86-specific user.

--Andy
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