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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