Re: [PATCH V6] powerpc/bug: Add ARCH_WARN_ASM and refactor _EMIT_BUG_ENTRY for Rust support

[Mukesh Kumar Chaurasiya]

On Thu, Sep 24, 2026 at 02:55:55PM +0200, Christophe Leroy (CS GROUP) wrote:
> 
> 
> Le 22/09/2026 à 07:43, 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 with
> 
> I'm a bit puzzled by the macro name. The name ARCH_WARN_ASM suggests that it
> is used for warnings like WARN() and/or WARN_ON(). Which means it should
> include flags BUGFLAG_WARNING and BUGFLAG_TAINT(TAINT_WARN).
> 
ARCH_WARN_ASM is a raw asm template used by the Rust warn infrastructure
(rust/kernel/bug.rs). It does not bake in BUGFLAG_WARNING or
BUGFLAG_TAINT(TAINT_WARN) — the caller (warn_flags! macro) is responsible
for ORing those into 'flags' before expanding this macro. This matches
the x86 convention where _BUG_FLAGS_ASM is also flags-agnostic.

> Also I have the feeling that it only handles WARN().
> WARN_ON() seems to have been forgotten.
> 
I might be wrong here but WARN_ON uses an unconditional trap and
requires a register operand in C side. I am not sure if rust can do
that.
> Christophe
> 

Regards,
Mukesh


> > ```
> > error: no rules expected `ARCH_WARN_ASM`
> >     --> /home/linkmauve/dev/linux/wii/rust/kernel/generated_arch_warn_asm.rs:1:28
> >      |
> >    1 | ::kernel::concat_literals!(ARCH_WARN_ASM("{file}", "{line}", "{flags}", "{size}"))
> >      |                            ^^^^^^^^^^^^^ no rules expected this token in macro call
> >      |
> >     ::: ../rust/kernel/lib.rs:279:1
> >      |
> > 279 | macro_rules! concat_literals {
> >      | ---------------------------- when calling this macro
> >      |
> >      = note: while trying to match sequence start
> > 
> > error: no rules expected `ARCH_WARN_REACHABLE`
> >     --> /home/linkmauve/dev/linux/wii/rust/kernel/generated_arch_reachable_asm.rs:1:28
> >      |
> >    1 | ::kernel::concat_literals!(ARCH_WARN_REACHABLE)
> >      |                            ^^^^^^^^^^^^^^^^^^^ no rules expected this token in macro call
> >      |
> >     ::: ../rust/kernel/lib.rs:279:1
> >      |
> > 279 | macro_rules! concat_literals {
> >      | ---------------------------- when calling this macro
> >      |
> >      = note: while trying to match sequence start
> > 
> > error: aborting due to 2 previous errors
> > ```
> > 
> > To add ARCH_WARN_ASM, _EMIT_BUG_ENTRY first needs to be refactored.
> > The old definition was a bare macro with no parameters, relying on
> > positional asm operand references (%0-%3), hardcoding the backward
> > reference to local label 1b, and including .org/.previous directives
> > inline. That made it impossible to compose as a plain string outside of
> > an asm operand context, and left an invisible contract that callers must
> > always emit their trap at label 1:.
> > 
> > Refactor _EMIT_BUG_ENTRY to take explicit (bug_entry, trap, file, line, flags)
> > string arguments via string concatenation. This removes the dependency
> > on asm operand numbering and makes the labels an explicit argument,
> > so the caller's intent is visible at the call site and a future caller
> > using a different label cannot silently produce a wrong bug table entry.
> > 
> > Move the .org and .previous directives out of _EMIT_BUG_ENTRY and into
> > each call site, so BUG_ENTRY() can still pass sizeof(struct bug_entry)
> > as an asm operand while ARCH_WARN_ASM can supply its own size string
> > 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.
> > 
> > Reported-by: FUJITA Tomonori <tomo@xxxxxxxxxxxx>
> > Closes: https://lore.kernel.org/all/anG67Q6Y59kDqh-c@desktop
> > Fixes: 73b741adb264 ("rust: Add PowerPC support")
> > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@xxxxxxxxx>
> > ---
> > Changelog:
> > V5 -> V6:
> >   - Dropped KUit tests from this series. It will be sent separately.
> > V5: https://lore.kernel.org/all/20260915090453.1227034-1-mkchauras@xxxxxxxxx/
> > 
> > V4 -> V5:
> >   - Fixed a build error with DEBUG_BUGVERBOSE=n
> >   - Added a label for bug entry
> > V4: https://lore.kernel.org/all/20260912065902.24017-1-mkchauras@xxxxxxxxx/
> > 
> > V3 -> V4:
> >   - Fix Label with appending b at end
> >   - Add KUnit test patch
> >   - Tested on ppc64le pseries: pass:5 fail:0 skip:0
> >   - Tested on ppc32 QEMU mac99 G4: pass:5 fail:0 skip:0
> >   - Tested on ppc64le QEMU pseries: pass:5 fail:0 skip:0
> > V3: https://lore.kernel.org/all/20260910100801.2159785-2-mkchauras@xxxxxxxxx
> > 
> > V2 -> V3:
> >   - Add label argument in _EMIT_BUG_ENTRY
> > V2: https://lore.kernel.org/all/20260910071252.1950488-2-mkchauras@xxxxxxxxx
> > 
> > V1 -> V2:
> >   - commit message now has error, fixes tag and closes tag
> > V1: https://lore.kernel.org/all/20260819084825.969116-1-mkchauras@xxxxxxxxx
> > 
> >   arch/powerpc/include/asm/bug.h | 46 ++++++++++++++++++----------------
> >   1 file changed, 25 insertions(+), 21 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> > index 0db48977c70c..bf31ee1e902a 100644
> > --- a/arch/powerpc/include/asm/bug.h
> > +++ b/arch/powerpc/include/asm/bug.h
> > @@ -32,34 +32,38 @@
> >   #endif /* verbose */
> >   #else /* !__ASSEMBLER__ */
> > -/* _EMIT_BUG_ENTRY expects args %0,%1,%2,%3 to be FILE, LINE, flags and
> > -   sizeof(struct bug_entry), respectively */
> >   #ifdef CONFIG_DEBUG_BUGVERBOSE
> > -#define _EMIT_BUG_ENTRY				\
> > -	".section __bug_table,\"aw\"\n"		\
> > -	"2:	.4byte 1b - .\n"		\
> > -	"	.4byte %0 - .\n"		\
> > -	"	.short %1, %2\n"		\
> > -	".org 2b+%3\n"				\
> > -	".previous\n"
> > +#define _EMIT_BUG_ENTRY(bug_entry, trap, file, line, flags)	\
> > +	".section __bug_table,\"aw\"\n"				\
> > +	#bug_entry ":	.4byte " #trap " - .\n"			\
> > +	"		.4byte " file " - .\n"			\
> > +	"		.short " line ", " flags "\n"
> >   #else
> > -#define _EMIT_BUG_ENTRY				\
> > -	".section __bug_table,\"aw\"\n"		\
> > -	"2:	.4byte 1b - .\n"		\
> > -	"	.short %2\n"			\
> > -	".org 2b+%3\n"				\
> > -	".previous\n"
> > +#define _EMIT_BUG_ENTRY(bug_entry, trap, file, line, flags)	\
> > +	".section __bug_table,\"aw\"\n"				\
> > +	#bug_entry ":	.4byte " #trap " - .\n"			\
> > +	"		.short " flags "\n"
> >   #endif
> > -#define BUG_ENTRY(cond_str, insn, flags, ...)		\
> > -	__asm__ __volatile__(				\
> > -		"1:	" insn "\n"			\
> > -		_EMIT_BUG_ENTRY				\
> > +#define BUG_ENTRY(cond_str, insn, flags, ...)			\
> > +	__asm__ __volatile__(					\
> > +		"1:	" insn "\n"				\
> > +		_EMIT_BUG_ENTRY(2, 1b, "%0", "%1", "%2")	\
> > +		".org 2b+%3\n"					\
> > +		".previous\n"					\
> >   		: : "i" (WARN_CONDITION_STR(cond_str) __FILE__), "i" (__LINE__),	\
> > -		  "i" (flags),				\
> > -		  "i" (sizeof(struct bug_entry)),	\
> > +		  "i" (flags),					\
> > +		  "i" (sizeof(struct bug_entry)),		\
> >   		  ##__VA_ARGS__)
> > +#define ARCH_WARN_ASM(file, line, flags, size)			\
> > +		"1:	twi 31, 0, 0\n"				\
> > +		_EMIT_BUG_ENTRY(2, 1b, file, line, flags)	\
> > +		".org 2b+" size "\n"				\
> > +		".previous\n"
> > +
> > +#define ARCH_WARN_REACHABLE
> > +
> >   /*
> >    * BUG_ON() and WARN_ON() do their best to cooperate with compile-time
> >    * optimisations. However depending on the complexity of the condition
>