linux-kernel - Re: x86: asm doubt

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Message-ID: <49BE7C57.6060100@goop.org>
Date:	Mon, 16 Mar 2009 09:20:39 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Sergio Luis <eeeesti@...il.com>, Ingo Molnar <mingo@...e.hu>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Pavel Machek <pavel@...e.cz>,
	Linux-kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86: asm doubt

Avi Kivity wrote:
> Jeremy Fitzhardinge wrote:
>> In 32-bit mode, moving to a segment register can fault if the 
>> underlying GDT/LDT entry is invalid.  In 64-bit mode, segment 
>> registers are mostly decorative and have no function, and moving 
>> arbitrary values into them doesn't fault, making the exception 
>> catching unnecessary.
>
> 64-bit mode will in fact fault if an access is made outside the 
> GDT/LDT, or of the descriptor is invalid.

Oh, I didn't think it bothered.  I guess I'm conflating that with the 
fact that it doesn't apply any of the segment properties to memory 
accesses.  Anyway, in that case the 64-bit code should definitely be 
using the standard segment accessors.

    J

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