linux-kernel - Assignment of GDT entries

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Date:	Wed, 13 Sep 2006 11:58:59 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...l.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <ak@...e.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Zachary Amsden <zach@...are.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michael A Fetterman <Michael.Fetterman@...cam.ac.uk>
Subject: Assignment of GDT entries

What's the rationale for the current assignment of GDT entries?  In 
particular, this section:

 *   0 - null
 *   1 - reserved
 *   2 - reserved
 *   3 - reserved
 *
 *   4 - unused			<==== new cacheline
 *   5 - unused
 *
 *  ------- start of TLS (Thread-Local Storage) segments:
 *
 *   6 - TLS segment #1			[ glibc's TLS segment ]
 *   7 - TLS segment #2			[ Wine's %fs Win32 segment ]
 *   8 - TLS segment #3
 *   9 - reserved
 *  10 - reserved
 *  11 - reserved

What are entries 1-3 and 9-11 reserved for?  Must they be unused for 
some reason, or is there some proposed use that has not been impemented yet?

Also, is there a particular reason kernel GDT entries start at 12?  
Would there be a problem in using either 4 or 5 for a kernel GDT descriptor?

I'm asking because I'd like to use one of these entries for the PDA 
descriptor, so that it is on the same cache line as the TLS 
descriptors.  That way, the entry/exit segment register reloads would 
still only need to touch two GDT cache lines.  Would there be a real 
problem in doing this?

Thanks,
    J

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