linux-kernel - Re: [PATCH RFC 3/6] Use %gs as the PDA base-segment in the kernel.

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Date:	Mon, 28 Aug 2006 10:51:00 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>, "Andi Kleen" <ak@...e.de>
Cc:	"Chuck Ebbert" <76306.1226@...puserve.com>,
	"H. J. Lu" <hjl@...on.org>, "Andrew Morton" <akpm@...l.org>,
	<linux-kernel@...r.kernel.org>, "Zachary Amsden" <zach@...are.com>
Subject: Re: [PATCH RFC 3/6] Use %gs as the PDA base-segment in the
	kernel.

>>> Andi Kleen <ak@...e.de> 27.08.06 20:19 >>>
>On Sunday 27 August 2006 19:20, Jeremy Fitzhardinge wrote:
>> Andi Kleen wrote:
>> >> +1:	movw GS(%esp), %gs
>> >>     
>> >
>> > movl is recommended in 32bit mode
>> >   
>> 
>> arch/i386/kernel/entry.S: Assembler messages:
>> arch/i386/kernel/entry.S:334: Error: suffix or operands invalid for `mov'
>
>Looks like a gas bug to me.

This was an intentional change (by H.J. if I recall right) as using movl
with segment registers gives the incorrect impression that one gets a
32-bit memory access (especially for stores this is important, since
there's really nothing stored to the upper 16 bits). One should always
use suffix-less 'mov' for segment register accesses.

Jan
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