linux-kernel - Re: [PATCH 01/11] riscv: add landing pad for asm routines.

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Message-ID: <2ba0c09d-2783-4dce-a889-06e16abbde61@canonical.com>
Date: Fri, 25 Jul 2025 08:13:29 +0200
From: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
To: Deepak Gupta <debug@...osinc.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 linux-kbuild@...r.kernel.org, linux-mm@...ck.org, llvm@...ts.linux.dev,
 rick.p.edgecombe@...el.com, broonie@...nel.org, cleger@...osinc.com,
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 bjorn@...osinc.com, fweimer@...hat.com, jeffreyalaw@...il.com,
 andrew@...ive.com, ved@...osinc.com, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
 Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
 Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor
 <nathan@...nel.org>, Nicolas Schier <nicolas.schier@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>,
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 Bill Wendling <morbo@...gle.com>, Monk Chiang <monk.chiang@...ive.com>,
 Kito Cheng <kito.cheng@...ive.com>, Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH 01/11] riscv: add landing pad for asm routines.

On 25.07.25 01:36, Deepak Gupta wrote:
> SYM_* macros are used to define assembly routines. In this patch series,
> re-define those macros in risc-v arch specific include file to include
> a landing pad instruction at the beginning. This is done only when the
> compiler flag for landing pad is enabled (i.e. __riscv_zicfilp).
> 
> TODO: Update `lpad 0` with `lpad %lpad_hash(name)` after toolchain
> support.
> 
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>   arch/riscv/include/asm/linkage.h | 42 ++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 42 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/linkage.h b/arch/riscv/include/asm/linkage.h
> index 9e88ba23cd2b..162774b81158 100644
> --- a/arch/riscv/include/asm/linkage.h
> +++ b/arch/riscv/include/asm/linkage.h
> @@ -6,7 +6,49 @@
>   #ifndef _ASM_RISCV_LINKAGE_H
>   #define _ASM_RISCV_LINKAGE_H
>   
> +#ifdef __ASSEMBLY__
> +#include <asm/assembler.h>
> +#endif
> +
>   #define __ALIGN		.balign 4
>   #define __ALIGN_STR	".balign 4"
>   
> +#ifdef __riscv_zicfilp
> +/*
> + * A landing pad instruction is needed at start of asm routines
> + * re-define macros for asm routines to have a landing pad at
> + * the beginning of function. Currently use label value of 0x1.

Your code below uses label value 0 which disables tag checking. As long 
as we don't have tool support for calculating function hashes that is an 
appropriate approach.

%s/Currently use label value of 0x1./Label value 0x0 disables tag checking./

Best regards

Heinrich

> + * Eventually, label should be calculated as a hash over function
> + * signature.
> + */
> +#define SYM_FUNC_START(name)				\
> +	SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)	\
> +	lpad 0;
> +
> +#define SYM_FUNC_START_NOALIGN(name)			\
> +	SYM_START(name, SYM_L_GLOBAL, SYM_A_NONE)	\
> +	lpad 0;
> +
> +#define SYM_FUNC_START_LOCAL(name)			\
> +	SYM_START(name, SYM_L_LOCAL, SYM_A_ALIGN)	\
> +	lpad 0;
> +
> +#define SYM_FUNC_START_LOCAL_NOALIGN(name)		\
> +	SYM_START(name, SYM_L_LOCAL, SYM_A_NONE)	\
> +	lpad 0;
> +
> +#define SYM_FUNC_START_WEAK(name)			\
> +	SYM_START(name, SYM_L_WEAK, SYM_A_ALIGN)	\
> +	lpad 0;
> +
> +#define SYM_FUNC_START_WEAK_NOALIGN(name)		\
> +	SYM_START(name, SYM_L_WEAK, SYM_A_NONE)		\
> +	lpad 0;
> +
> +#define SYM_TYPED_FUNC_START(name)				\
> +	SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)	\
> +	lpad 0;
> +
> +#endif
> +
>   #endif /* _ASM_RISCV_LINKAGE_H */
>