linux-kernel - Re: Do we still care about compilers without __seg_fs and __seg

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Message-ID: <CAFULd4a=LPW0z5wUx1uSq2J9A9DmkO+368XCrkzAw7GgWGoMUg@mail.gmail.com>
Date: Sat, 20 Dec 2025 08:50:57 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	"x86@...nel.org" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Do we still care about compilers without __seg_fs and __seg_gs support??

On Sat, Dec 20, 2025 at 1:48 AM H. Peter Anvin <hpa@...or.com> wrote:
>
> On 2025-12-19 16:24, Nathan Chancellor wrote:
> > On Fri, Dec 19, 2025 at 03:24:21PM -0800, H. Peter Anvin wrote:
> >> As of Linux 6.16, we require:
> >>
> >>      gcc 8.1 or higher
> >>      clang 15.0.0 or higher
> >>
> >> If my reading of the release notes is correct, then both versions *should*
> >> supported __seg_fs and __seg_gs, but we have:
> >>
> >> config CC_HAS_NAMED_AS
> >>         def_bool $(success,echo 'int __seg_fs fs; int __seg_gs gs;' | $(CC) -x
> >> c - -S -o /dev/null)
> >>         depends on CC_IS_GCC
> >>
> >> We don't even try on clang.
> >>
> >> Being able to actually rely on the compiler for this would make a lot of
> >> things cleaner.  For one thing, I'm trying to untangle a bunch of ugliness in
> >> the code sharing between realmode and proper flat mode code...
> >>
> >> Uros, you seem to have touched this code as recently as earlier this year; any
> >> thoughts?
> >>
> >> What about the LLVM people, any insights?
> >
> > Trying to use __seg_fs or __seg_gs in certain cases crashes the X86
> > backend.
> >
> >   https://github.com/llvm/llvm-project/issues/93449
> >
> > Is there anyone on AMD or Intel's LLVM teams that could look into
> > solving that? Nick pinged a couple of Intel's folks but it does not look
> >
>
> It looks to me that this is specifically related to static initializers, or is
> there something else here that I'm not sure about?
>
> I'm asking because it might still allow at least the boot code improvements,
> and/or have some other less painful workaround than carrying these hacks.
>
> As far as I can tell, on x86 gcc will not change the value of the pointer when
> cast to a different address space and I believe Linux expects this behavior.

This is because "... these address spaces are not considered to be
subspaces of the generic (flat) address space. This means that
explicit casts are required to convert pointers between these address
spaces and the generic address space. In practice the application
should cast to uintptr_t and apply the segment base offset that it
installed previously." [1]

[1] https://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html

Uros.