[PATCH v3] riscv: asm: use .insn for making custom instructions

[Andrew Jones]

On Thu, Oct 30, 2025 at 11:10:09PM -0400, Nathan Chancellor wrote:
> Hey Ben,
> 
> On Fri, Oct 24, 2025 at 06:16:40PM +0100, Ben Dooks wrote:
> > The assembler has .insn for building custom instructions
> > now, so change the .4byte to .insn. This ensures the output
> > is marked as an instruction and not as data which may
> > confuse both debuggers and anything else that relies on
> > this sort of marking.
> > 
> > Add an ASM_INSN_I() wrapper in asm.h to allow the selecting
> > of how this is output so older assemblers are still good.
> > 
> > Reviewed-by: Andrew Jones <ajones at ventanamicro.com>
> > Signed-off-by: Ben Dooks <ben.dooks at codethink.co.uk>
> > ---
> > v2:
> >   - fixed #ifndef v #ifdef
> >   - added custom MIPS instructions too
> >   - new patch description
> > v3:
> >   - fixed typo missed in compile test of NOP4
> >   - fixed incorrect MIPS changes
> >   - re-wored slightly to shorten sentences and remove typos
> > ---
> >  arch/riscv/include/asm/asm.h                    | 6 ++++++
> >  arch/riscv/include/asm/insn-def.h               | 8 ++++----
> >  arch/riscv/include/asm/vendor_extensions/mips.h | 6 +++---
> >  3 files changed, 13 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
> > index 8bd2a11382a3..2d97436999ca 100644
> > --- a/arch/riscv/include/asm/asm.h
> > +++ b/arch/riscv/include/asm/asm.h
> > @@ -12,6 +12,12 @@
> >  #define __ASM_STR(x)	#x
> >  #endif
> >  
> > +#ifdef CONFIG_AS_HAS_INSN
> 
> The current AS_HAS_INSN check in Kconfig is not sufficient for this
> usage. With LLVM prior to [1] (i.e., older than 19) or GNU binutils
> prior to [2] (i.e., older than 2.38), the build breaks:
> 
>   In file included from <built-in>:4:
>   In file included from lib/vdso/gettimeofday.c:6:
>   In file included from include/vdso/datapage.h:21:
>   In file included from include/vdso/processor.h:10:
>   arch/riscv/include/asm/vdso/processor.h:23:2: error: expected instruction format
>      23 |         ALT_RISCV_PAUSE();
>         |         ^
>   arch/riscv/include/asm/errata_list.h:47:3: note: expanded from macro 'ALT_RISCV_PAUSE'
>      47 |                 RISCV_PAUSE, /* Original RISC‑V pause insn */ \
>         |                 ^
>   arch/riscv/include/asm/insn-def.h:259:21: note: expanded from macro 'RISCV_PAUSE'
>     259 | #define RISCV_PAUSE     ASM_INSN_I("0x100000f")
>         |                         ^
>   arch/riscv/include/asm/asm.h:16:26: note: expanded from macro 'ASM_INSN_I'
>      16 | #define ASM_INSN_I(__x) ".insn " __x
>         |                          ^
>   <inline asm>:5:7: note: instantiated into assembly here
>       5 | .insn 0x100000f
>         |       ^
> 
>   arch/riscv/include/asm/vdso/processor.h: Assembler messages:
>   arch/riscv/include/asm/vdso/processor.h:27: Error: unrecognized opcode `0x100000f'
>   arch/riscv/include/asm/vdso/processor.h:27: Error: unrecognized opcode `0x100000f'
>   arch/riscv/include/asm/vdso/processor.h:27: Error: unrecognized opcode `0x100000f'
>   arch/riscv/include/asm/vdso/processor.h:27: Error: unrecognized opcode `0x100000f'
>   make[4]: *** [scripts/Makefile.build:287: arch/riscv/kernel/vdso/vgettimeofday.o] Error 1
> 
> Maybe this wants a new symbol like CONFIG_AS_HAS_INSN_EXT that checks
> for this support?
> 
> [1]: https://github.com/llvm/llvm-project/commit/2a086dce691e3cc34a2fc27f4fb255bb2cbbfac9
> [2]: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=a262b82fdbf4cda3b0648b1adc32245ca3f78b7a

Or we can change the test in 'config AS_HAS_INSN' to use the hex encoded
input. We'd completely stop using .insn for LLVM older than 19 and GNU
binutils older than 2.38, but that's probably better than adding yet
another config.

Thanks,
drew

> 
> Cheers,
> Nathan
> 
> > +#define ASM_INSN_I(__x) ".insn " __x
> > +#else
> > +#define ASM_INSN_I(__x) ".4byte " __x
> > +#endif
> > +
> >  #if __riscv_xlen == 64
> >  #define __REG_SEL(a, b)	__ASM_STR(a)
> >  #elif __riscv_xlen == 32
> > diff --git a/arch/riscv/include/asm/insn-def.h b/arch/riscv/include/asm/insn-def.h
> > index c9cfcea52cbb..d29da6ccd3dd 100644
> > --- a/arch/riscv/include/asm/insn-def.h
> > +++ b/arch/riscv/include/asm/insn-def.h
> > @@ -256,10 +256,10 @@
> >  	INSN_S(OPCODE_OP_IMM, FUNC3(6), __RS2(3),		\
> >  	       SIMM12((offset) & 0xfe0), RS1(base))
> >  
> > -#define RISCV_PAUSE	".4byte 0x100000f"
> > -#define ZAWRS_WRS_NTO	".4byte 0x00d00073"
> > -#define ZAWRS_WRS_STO	".4byte 0x01d00073"
> > -#define RISCV_NOP4	".4byte 0x00000013"
> > +#define RISCV_PAUSE	ASM_INSN_I("0x100000f")
> > +#define ZAWRS_WRS_NTO	ASM_INSN_I("0x00d00073")
> > +#define ZAWRS_WRS_STO	ASM_INSN_I("0x01d00073")
> > +#define RISCV_NOP4	ASM_INSN_I("0x00000013")
> >  
> >  #define RISCV_INSN_NOP4	_AC(0x00000013, U)
> >  
> > diff --git a/arch/riscv/include/asm/vendor_extensions/mips.h b/arch/riscv/include/asm/vendor_extensions/mips.h
> > index ea8ca747d691..ffeb12dc17a3 100644
> > --- a/arch/riscv/include/asm/vendor_extensions/mips.h
> > +++ b/arch/riscv/include/asm/vendor_extensions/mips.h
> > @@ -30,8 +30,8 @@ extern struct riscv_isa_vendor_ext_data_list riscv_isa_vendor_ext_list_mips;
> >   * allowing any subsequent instructions to fetch.
> >   */
> >  
> > -#define MIPS_PAUSE	".4byte 0x00501013\n\t"
> > -#define MIPS_EHB	".4byte 0x00301013\n\t"
> > -#define MIPS_IHB	".4byte 0x00101013\n\t"
> > +#define MIPS_PAUSE	ASM_INSN_I("0x00501013\n\t")
> > +#define MIPS_EHB	ASM_INSN_I("0x00301013\n\t")
> > +#define MIPS_IHB	ASM_INSN_I("0x00101013\n\t")
> >  
> >  #endif // _ASM_RISCV_VENDOR_EXTENSIONS_MIPS_H
> > -- 
> > 2.37.2.352.g3c44437643
> > 
> > 
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