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

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Message-ID: <CA+55aFzU=zvjBPRSKCYHdf3ET-oq6s66e_PHHGcDwf3sJx+j+A@mail.gmail.com>
Date:	Tue, 12 Feb 2013 15:49:07 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	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. Peter Anvin" <hpa@...ux.intel.com>,
	"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 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".

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.

              Linus
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