Re: Assignment of GDT entries

Linus Torvalds wrote:
These _used_ to be the "user CS/DS" respectively, but that got changed around by me when did the "sysenter" support.

So does this mean that moving the user-visible cs/ds isn't likely to break stuff, if it has been done before?

The sysenter logic (or, more properly, the sysexit one) requires that the user code segment number is the same as the kernel code segment +2 (ie "+16" in actual selector term). And the user data segment needs to be +3.

Yep, I'm aware of that constraint.

And segment #8 (ie 0x40) is special (TLS segment #3), of course. Anybody who wants to emulate windows or use the BIOS needs to use that for their "common BIOS area" thing, iirc.

Do you mean that something like dosemu/Wine needs to be able to use GDT #8? Or is it only used in kernel code?

See above. The kernel and user segments have to be moved as a block of four, and obviously we'd like to keep them in the same cacheline too. Also, the cacheline that contains segment #8/0x40 is not available,

Why's that? That cacheline (assuming 64 byte line size) already contains the user/kernel/cs/ds descriptors.

I'm thinking of putting together a patch to change the descriptor use to:

8 - TLS #1
9 - TLS #2
10 - TLS #3
11 - Kernel PDA
12 - Kernel CS
13 - Kernel DS
14 - User CS
15 - User DS


This has the advantage of leaving the user cs/ds unchanged. From what people had said so far, this should be OK, other than making the heavily used TLS #1 share the BIOS common area entry number. If this needs to be usable by userspace for something special, then making it TLS #1 won't fly...

Alternatively, maybe:

0 - NULL
1 - Kernel PDA
2 - Kernel CS
3 - Kernel DS
4 - User CS
5 - User DS
6 - TLS #1
7 - TLS #2


which moves the user cs/ds, but avoids #8.

J
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