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Date: Wed, 3 Apr 2024 10:57:23 -0700
From: Nathan Chancellor <nathan@kernel.org>
To: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	kernel test robot <lkp@intel.com>, oe-kbuild-all@lists.linux.dev,
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	Arjan van de Ven <arjan@linux.intel.com>, x86@kernel.org,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Sparse Mailing-list <linux-sparse@vger.kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>, llvm@lists.linux.dev
Subject: Re: arch/x86/include/asm/processor.h:698:16: sparse: sparse:
 incorrect type in initializer (different address spaces)
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On Tue, Apr 02, 2024 at 01:43:00PM +0200, Uros Bizjak wrote:
> On Mon, Mar 4, 2024 at 8:07 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > On Mon, Mar 04 2024 at 06:42, Uros Bizjak wrote:
> >
> > > On Mon, Mar 4, 2024 at 12:49 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> > >>
> > >> On Sun, Mar 03 2024 at 21:24, Uros Bizjak wrote:
> > >> > On Sun, Mar 3, 2024 at 9:21 PM Uros Bizjak <ubizjak@gmail.com> wrote:
> > >> >> On Sun, Mar 3, 2024 at 9:10 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > >> >> > That's so sad because it would provide us compiler based __percpu
> > >> >> > validation.
> > >> >>
> > >> >> Unfortunately, the c compiler can't strip qualifiers, so typeof() is
> > >> >> of limited use also when const and volatile qualifiers are used.
> > >> >> Perhaps some extension could be introduced to c standard to provide an
> > >> >> unqualified type, e.g. typeof_unqual().
> > >> >
> > >> > Oh, there is one in C23 [1].
> > >>
> > >> Yes. I found it right after ranting.
> > >>
> > >> gcc >= 14 and clang >= 16 have support for it of course only when adding
> > >> -std=c2x to the command line.
> > >>
> > >> Sigh. The name space qualifiers are non standard and then the thing
> > >> which makes them more useful is hidden behind a standard.
> > >
> > > With GCC, you can use __typeof_unqual__ (please note underscores)
> > > without -std=c2x [1]:
> > >
> > > "... Alternate spelling __typeof_unqual__ is available in all C modes
> > > and provides non-atomic unqualified version of what __typeof__
> > > operator returns..."
> > >
> > > Please also see the example in my last post. It can be compiled without -std=...
> >
> > With gcc >= 14. Not so with clang...
> 
> Please note that clang-17.0.6 currently fails to compile kernel with
> named address spaces [1]. So perhaps kernel can use __typeof_unqual__
> (available without -std=c2x) in the hope that clang implements
> __typeof_unqual__ in one of its next releases, following the examples
> of GCC [2] and MSVC[3].

This is now supported in clang 19.0.0 (main):

https://github.com/llvm/llvm-project/commit/cc308f60d41744b5920ec2e2e5b25e1273c8704b

I have inquired about applying this to the 18.x series, such that it
would either make 18.1.3 or 18.1.4, but that is still open for
discussion.

I think the error that I mentioned at [1] is resolved with using
__typeof_unqual__, I tested this diff, which is likely incorrect but
allows me to continue testing without that warning/error due to -Werror:

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index 20696df5d567..fc77c99d2e80 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -95,7 +95,7 @@
 
 #endif /* CONFIG_SMP */
 
-#define __my_cpu_type(var)	typeof(var) __percpu_seg_override
+#define __my_cpu_type(var)	__typeof_unqual__(var) __percpu_seg_override
 #define __my_cpu_ptr(ptr)	(__my_cpu_type(*ptr)*)(__force uintptr_t)(ptr)
 #define __my_cpu_var(var)	(*__my_cpu_ptr(&(var)))
 #define __percpu_arg(x)		__percpu_prefix "%" #x

However, I get a crash in LLVM's backend with that diff applied on top
of commit 034dd140a6d8 ("Merge branch into tip/master: 'x86/shstk'"),
which appears to be another tangential issue. I've filed
https://github.com/ClangBuiltLinux/linux/issues/2013 so that we don't
lose track of this.

Cheers,
Nathan