linux-kernel - Re: [tip:x86/fpu] x86-64, asm: Use fxsaveq/fxrestorq in more places

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Date:	Fri, 22 Oct 2010 20:39:55 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	mingo@...hat.com, linux-kernel@...r.kernel.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/fpu] x86-64, asm: Use fxsaveq/fxrestorq in more places

On 10/22/2010 07:11 PM, Linus Torvalds wrote:
> On Fri, Oct 22, 2010 at 6:20 PM, tip-bot for H. Peter Anvin
> <hpa@...or.com> wrote:
>>
>> Checkin d7acb92fea932ad2e7846480aeacddc2c03c8485 made use of fxsaveq
>> in fpu_fxsave() if the assembler supports it; this adds
>> fxsaveq/fxrstorq to fxrstor_checking() and fxsave_user() as well.
> 
> Hmm. I really think you should be able to do something like this
> 
>   #ifdef CONFIG_AS_FXSAVEQ
>     #define FXSAVEQ_INSN "fxsaveq %[fx]"
>     #define FXRSTORQ_INSN "fxrstorq %[fx]"
>     #define FX_EXTRA(ptr)
>   #else
>     #define FXSAVEQ_INSN "rex64/fxsave (%[fxr])"
>     #define FXRSTORQ_INSN "rex64/fxrstor (%[fxr])"
>     #define FX_EXTRA(ptr)  [fxr] "R" (ptr)
>   #endif
> 
> and then you should be able to write the actual code something like
> 
>    asm volatile(FXSAVEQ_INSN
>       :[fx] "=m" (memory)
>       :FX_EXTRA(&memory))
> 
> and
> 
>   #define __COMMA_CONCAT(x) ,##x
>   #define COMMA_CONCAT(x) __COMMA_CONCAT(x)
>   asm volatile(FXRSTRORQ_INSN
>       : :[fx] "m" (memory) COMMA_CONCAT(FX_EXTRA(&memory)));
> 
> or something like that (obviously you'll need the exception stuff too there).
> 
> With no #ifdef's in the actual code. Hmm?
> 

Well sure, but it is already encapsulated in 3 inlines.  To me, the
above is quite frankly quite a bit harder to read than the broken-out form.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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