linux-kernel - Re: [tip:x86/mm] x86, mm: Redesign get_user with a __builtin_choose

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Date:	Tue, 12 Feb 2013 15:52:52 -0800
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jamie Lokier <jamie@...reable.org>,
	ville.syrjala@...ux.intel.com, Borislav Petkov <bp@...en8.de>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H.J. Lu" <hjl.tools@...il.com>, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mm] x86, mm: Redesign get_user with a __builtin_choose_expr
 hack

On 02/12/2013 03:49 PM, Linus Torvalds wrote:
> On Tue, Feb 12, 2013 at 3:19 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> Yes, but there doesn't seem to be any other way to do this.  gcc won't
>> even allow "=cd" even if we know the variable is 64 bits, even though
>> "=A" is documented to be equivalent to "=da".
> 
> No, "=da" means value "in edx _or_ %eax". Not the same as "A".
> 

Actually, if you look at how gcc implements them, they are the same, and
if you are luckless enough to try to use a 32-bit value with an "A"
constraint you have it end up in either %eax or %edx.

However, they seem to have added some additional linting which prohibits
the compound form.  I'm not sure it would have worked anyway since we
need the two-register bit to be conditional.

> But you're right, there's nothing similar for %ebx:%ecx. I thought
> there was. I was really sure we did something special for 64-bit adc
> etc.
> 
>> Let me know what you think.
> 
> I guess we don't have any choice. And the other cleanups certainly look good.

OK, will commit the comment.  We can add the additional copy if we need it.

	-hpa


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