linux-kernel - Re: Assignment of GDT entries

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Date:	Wed, 13 Sep 2006 13:20:46 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Linus Torvalds <torvalds@...l.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <ak@...e.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Zachary Amsden <zach@...are.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michael A Fetterman <Michael.Fetterman@...cam.ac.uk>
Subject: Re: Assignment of GDT entries

Arjan van de Ven wrote:
> I don't know the exact details on these; I do know that several GDT
> entries tend to be used by BIOSes in their APM implementations and thus
> are better of not being used. That might be the underlying reason
> here....
>   

Hm, I see.

Also, thinking about this a bit more, it would be most helpful to move 
the PDA descriptor onto the same cache line as the other descriptors 
used in the kernel - ie, somewhere in the range of 8-15 (assuming 64 
byte line size):

 *   8 - TLS segment #3
 *   9 - reserved
 *  10 - reserved
 *  11 - reserved
 *
 *  ------- start of kernel segments:
 *
 *  12 - kernel code segment
 *  13 - kernel data segment
 *  14 - default user CS
 *  15 - default user DS

This seems pretty wasteful of the GDT cache line, since the kernel+user 
cs/ds are shared a cache line with 3 reserved entries and the never-used 
TLS #3 descriptor.    If it were OK to put the PDA in one of 9,10,11, 
then that would be good.  Unfortunately the next cache line is clogged 
up with PNP and APM stuff, which I presume not movable.

In fact, if we assume that "reserved" means "unusable", it looks like 
none of the GDT's cache lines can be freed up to lay out the most 
commonly used descriptors into a single cache line:

    line 0: NULL descriptor, 3 reserved, 2 unused, 2 TLS
    line 1: 1 TLS, 3 reserved, kernel+user code+data
    line 2: TSS, LDT, PNPBIOS, APMBIOS
    line 3: APMBIOS, ESPFIX, 4 unused, doublefault TSS

Otherwise line 1 would be ideal for putting 3 TLS, kernel+user code+data 
and PDA into, thereby making 99.999% of GDT descriptor uses come from 
one cache line.

But anyway, what breaks if I put the PDA in 11?

    J
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