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

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Message-ID: <c5432e6b-5a8d-4ef4-9cd4-c18e6a2b1e4b@zytor.com>
Date: Fri, 19 Dec 2025 15:24:21 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Uros Bizjak <ubizjak@...il.com>, Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
        Bill Wendling <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>
Cc: "x86@...nel.org" <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Do we still care about compilers without __seg_fs and __seg_gs
 support??

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?

	-hpa