linux-kernel - Re: [PATCH 1/3] x86: Move TSS and LDT to end of the GDT

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Date: Sat, 16 Dec 2023 19:24:35 +0100
From: Vegard Nossum <vegard.nossum@...cle.com>
To: Linus Torvalds <torvalds@...uxfoundation.org>,
        Brian Gerst <brgerst@...il.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
        Ingo Molnar <mingo@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/3] x86: Move TSS and LDT to end of the GDT


On 13/12/2023 20:08, Linus Torvalds wrote:
> I guess it's much too much work to really fix things, but maybe we
> could at least add #defines and comments for the special values.
> 
> So instead of
> 
>          GDT_ENTRY_INIT(0xc093, 0, 0xfffff)
> 
> we could maybe have
> 
>         #define GDT_ENTRY_FLAGS(type,s,dpl,p,avl,l,d,g) \
>                  ((type) |
>                   (s)<<4) | \
>                  (dpl) << 5) | ....
> 
> and have #defines for those 0xc093 values (with comments), so that we'd have
> 
>          GDT_ENTRY_INIT(KERNEL_DATA_FLAGS, 0, 0xffff)
> 
> instead of a magic 0xc093 number.
> 
> This would require some nit-picky "read all those values and know the
> crazy descriptor table layout" thing. Maybe somebody has a serious
> case of insomnia and boredom?

I took a stab at this, see attached RFC patch. Maybe this does too much,
though.

I did basic build and boot tests on both 64- and 32-bit, but I would
also try a binary diff before/after just to verify nothing changed by
accident, as well as making sure all the code is actually compiled (some
of the BIOS stuff only gets used on 32-bit with ISA/PNP enabled, for
example).

While preparing the patch I also came across some things that are
unclear to me:

- why do we want some segments with the A (accessed) bit set and some
with it cleared -- is there an actual reason for the difference, or
could we just set it for all of them?

- why does setup_percpu_segment() want the DB (size) flag clear? This
seems to indicate that it's a 16-bit segment -- is this correct?


Vegard
View attachment "0001-x86-replace-magic-numbers-in-GDT-descriptors.patch" of type "text/x-patch" (22004 bytes)