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

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Message-ID: <aIOQTSVNkC1RztDW@debug.ba.rivosinc.com>
Date: Fri, 25 Jul 2025 07:10:21 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Heinrich Schuchardt <heinrich.schuchardt@...onical.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, samitolvanen@...gle.com,
	apatel@...tanamicro.com, ajones@...tanamicro.com,
	conor.dooley@...rochip.com, charlie@...osinc.com,
	samuel.holland@...ive.com, 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>,
	David Hildenbrand <david@...hat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	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 Fri, Jul 25, 2025 at 08:13:29AM +0200, Heinrich Schuchardt wrote:
>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.
>

Yes I made the fix at other place where function prototype was determined
to be static (see `call_on_irq_stack` in entry.S)

In this patch, it wasn't possible.

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

Thanks its lingering from earlier. Will fix it.

>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 */
>>
>