Re: [PATCH] powerpc/bug: Add ARCH_WARN_ASM and refactor _EMIT_BUG_ENTRY for Rust support

On Wed, Aug 19, 2026 at 01:50:43PM +0200, Christophe Leroy (CS GROUP) wrote:
> 
> 
> Le 19/08/2026 à 10:48, Mukesh Kumar Chaurasiya (IBM) a écrit :
> > The Rust kernel infrastructure generates inline asm for WARN() via
> > ARCH_WARN_ASM(file, line, flags, size), expanding it through a C
> > preprocessor pass (generated_arch_warn_asm.rs.S) to produce an
> > arch-specific asm template string for use in Rust's core::arch macros.
> > 
> > powerpc currently lacks ARCH_WARN_ASM and ARCH_WARN_REACHABLE, causing
> > Rust builds to fail on powerpc.
> > 
> > Refactor _EMIT_BUG_ENTRY to accept explicit (file, line, flags) string
> > arguments rather than relying on positional asm operand references
> > (%0, %1, %2, %3). This allows the macro to be composed as a plain
> > string concatenation, which is required for ARCH_WARN_ASM where no asm
> > operand context exists.
> > 
> > Move the .org and .previous directives out of _EMIT_BUG_ENTRY and into
> > the BUG_ENTRY() call site to preserve existing behaviour while enabling
> > ARCH_WARN_ASM to supply its own size operand independently.
> > 
> > Add ARCH_WARN_REACHABLE as an empty define, matching the arm64
> > convention, indicating that no additional reachability annotation is
> > needed after a WARN on powerpc.
> > 
> > This brings powerpc into line with x86, arm64, s390, and riscv, all of
> > which already define ARCH_WARN_ASM and ARCH_WARN_REACHABLE.
> > 
> > Suggested-by: FUJITA Tomonori <[email protected]>
> > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <[email protected]>
> > ---
> >   arch/powerpc/include/asm/bug.h | 28 ++++++++++++++++------------
> >   1 file changed, 16 insertions(+), 12 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> > index 0db48977c70c..8aba39e0cf26 100644
> > --- a/arch/powerpc/include/asm/bug.h
> > +++ b/arch/powerpc/include/asm/bug.h
> > @@ -32,34 +32,38 @@
> >   #endif /* verbose */
> >   #else /* !__ASSEMBLER__ */
> 
> Sorry, I still don't understand. The only place the new macro is used is a
> .S file (namely rust/kernel/generated_arch_warn_asm.rs.S), and the change
> this patch implements is inside a #if !__ASSEMBLER__.
> 
> What am I missing ?
> 
> Christophe

Hey Christophe,

The code calling ARCH_WARN_ASM is a .rs.S file that goes through the C
preprocessor as part of the Rust build machinery.

The intended mechanism is essentially

generated_arch_warn_asm.rs.S
          |
          v
         CPP
          |
          | includes <linux/bug.h>
          |
          v
ARCH_WARN_ASM(...) gets expanded
          |
          v
Rust source containing the generated assembly string

The patch therefore needs ARCH_WARN_ASM in the non-__ASSEMBLER__ section,
because that section is where the macros intended for C preprocessing
are defined.

The thing is .rs.S is not using ARCH_WARN_ASM as an assembler-time macro,
it is using the C preprocessor to expand ARCH_WARN_ASM before the
Rust/assembly processing stage.

This is my understanding of things, if i am wrong somewhere maybe
someone can clarify.

Regards,
Mukesh


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