Messages in this thread | | | Date | Wed, 13 Sep 2006 14:47:36 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: Assignment of GDT entries |
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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.
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