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@xxxxxxxxxxxxx> wrote: > > > > On Mon, Mar 04 2024 at 06:42, Uros Bizjak wrote: > > > > > On Mon, Mar 4, 2024 at 12:49 AM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote: > > >> > > >> On Sun, Mar 03 2024 at 21:24, Uros Bizjak wrote: > > >> > On Sun, Mar 3, 2024 at 9:21 PM Uros Bizjak <ubizjak@xxxxxxxxx> wrote: > > >> >> On Sun, Mar 3, 2024 at 9:10 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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
