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

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Message-ID: <20060828145436.GA5710@lucon.org>
Date:	Mon, 28 Aug 2006 07:54:36 -0700
From:	"H. J. Lu" <hjl@...on.org>
To:	Jan Beulich <jbeulich@...ell.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>, Andi Kleen <ak@...e.de>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	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.

On Mon, Aug 28, 2006 at 10:51:00AM +0100, Jan Beulich wrote:
> >>> 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.

mov will generate the optimal opcode with old and new assemblers.


H.J.
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