Jeremy Fitzhardinge wrote:
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
TLS #3 overlaps BIOS 0x40, but code which calls borken APM / PnP BIOS
and sets up protected mode 0x40 GDT segment does so by swapping out the
TLS segment with the identity simulation of physical 0x400 offset,
swapping it back afterwards. Short of bugs in that code (which there
are, btw), you shouldn't need to be concerned with it.
I believe 9,10,11 are reserved for future users like yourself or
expanded TLS segments. I think a bank of 3 TLS segments in the GDT is
working fine now (does NPTL even use more than one?).
good. Unfortunately the next cache line is clogged up with PNP and
APM stuff, which I presume not movable.
Totally movable, actually, just means breaking module dependencies.
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.
That change is visible to userspace, unfortunately.
But anyway, what breaks if I put the PDA in 11?
Nothing.
Zach
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