linux-kernel - [RFC PATCH 3/3] LoongArch: Handle table jump option for RUST

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Message-ID: <20250903095315.15057-4-yangtiezhu@loongson.cn>
Date: Wed,  3 Sep 2025 17:53:15 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>
Cc: WANG Rui <wangrui@...ngson.cn>,
	rust-for-linux@...r.kernel.org,
	loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH 3/3] LoongArch: Handle table jump option for RUST

When compiling with LLVM and CONFIG_RUST is set, there exist objtool
warnings "sibling call from callable instruction with modified stack
frame" in rust/core.o and rust/kernel.o.

For this special case, the related object file shows that there is no
generated relocation section '.rela.discard.tablejump_annotate' for the
table jump instruction jirl, thus objtool can not know that what is the
actual destination address.

If the rustc has the option "-Cllvm-args=--loongarch-annotate-tablejump",
pass the option to enable jump table for objtool, otherwise it is better
to remain -Zno-jump-tables to keep compatibility with older rustc.

How to test:

(1) Please install rustc 1.78.0 (without annotate-tablejump option) or
1.87.0 (with annotate-tablejump option), do not use the latest version
for now, otherwise there may be build error:

     --> rust/kernel/lib.rs:331:13
      |
  331 |         loc.file_with_nul()
      |             ^^^^^^^^^^^^^ method not found in `&'a Location<'a>`

  error: aborting due to 1 previous error

(2) Execute the following command:

  $ rustup component add rust-src
  $ make LLVM=1 rustavailable
  $ make ARCH=loongarch LLVM=1 clean defconfig
  $ scripts/config -d MODVERSIONS \
    -e RUST -e SAMPLES -e SAMPLES_RUST \
    -e SAMPLE_RUST_CONFIGFS -e SAMPLE_RUST_MINIMAL \
    -e SAMPLE_RUST_MISC_DEVICE -e SAMPLE_RUST_PRINT \
    -e SAMPLE_RUST_DMA -e SAMPLE_RUST_DRIVER_PCI \
    -e SAMPLE_RUST_DRIVER_PLATFORM -e SAMPLE_RUST_DRIVER_FAUX \
    -e SAMPLE_RUST_DRIVER_AUXILIARY -e SAMPLE_RUST_HOSTPROGS
  $ make ARCH=loongarch LLVM=1 olddefconfig all

Suggested-by: WANG Rui <wangrui@...ngson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
---
 arch/loongarch/Kconfig  | 4 ++++
 arch/loongarch/Makefile | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index f0abc38c40ac..57933a717e92 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -298,6 +298,10 @@ config AS_HAS_LVZ_EXTENSION
 config CC_HAS_ANNOTATE_TABLEJUMP
 	def_bool $(cc-option,-mannotate-tablejump)
 
+config RUSTC_HAS_ANNOTATE_TABLEJUMP
+	depends on RUST
+	def_bool $(rustc-option,-Cllvm-args=--loongarch-annotate-tablejump)
+
 menu "Kernel type and options"
 
 source "kernel/Kconfig.hz"
diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index 9d80af7f75c8..b26d47707031 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -106,6 +106,12 @@ KBUILD_CFLAGS			+= -mannotate-tablejump
 else
 KBUILD_CFLAGS			+= -fno-jump-tables # keep compatibility with older compilers
 endif
+ifdef CONFIG_RUSTC_HAS_ANNOTATE_TABLEJUMP
+# Pass '--loongarch-annotate-tablejump' via '-Cllvm-args' to rustc when RUST is enabled.
+KBUILD_RUSTFLAGS		+= -Cllvm-args=--loongarch-annotate-tablejump
+else
+KBUILD_RUSTFLAGS		+= -Zno-jump-tables # keep compatibility with older compilers
+endif
 ifdef CONFIG_LTO_CLANG
 # The annotate-tablejump option can not be passed to LLVM backend when LTO is enabled.
 # Ensure it is aware of linker with LTO, '--loongarch-annotate-tablejump' also needs to
-- 
2.42.0