[PATCH 2/2] RISC-V: re-enable gcc + rust builds

[Asuna Yang <spriteovo@gmail.com>]

Commit 33549fcf37ec ("RISC-V: disallow gcc + rust builds") disabled GCC
+ Rust builds for RISC-V due to differences in extension handling
compared to LLVM.

Add a Kconfig non-visible symbol to ensure that all important RISC-V
specific flags that will be used by GCC can be correctly recognized by
Rust bindgen's libclang, otherwise config HAVE_RUST will not be
selected.

Signed-off-by: Asuna Yang <SpriteOvO@gmail.com>
---
 Documentation/rust/arch-support.rst |  2 +-
 arch/riscv/Kconfig                  | 62 ++++++++++++++++++++++++++++-
 rust/Makefile                       |  7 +++-
 3 files changed, 68 insertions(+), 3 deletions(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index 6e6a515d0899..5282e0e174e8 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -18,7 +18,7 @@ Architecture   Level of support  Constraints
 ``arm``        Maintained        ARMv7 Little Endian only.
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        \-
-``riscv``      Maintained        ``riscv64`` and LLVM/Clang only.
+``riscv``      Maintained        ``riscv64`` only.
 ``um``         Maintained        \-
 ``x86``        Maintained        ``x86_64`` only.
 =============  ================  ==============================================
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 1c5544401530..d7f421e0f429 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -195,7 +195,7 @@ config RISCV
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RETHOOK if !XIP_KERNEL
 	select HAVE_RSEQ
-	select HAVE_RUST if RUSTC_SUPPORTS_RISCV && CC_IS_CLANG
+	select HAVE_RUST if RUSTC_SUPPORTS_RISCV && RUST_BINDGEN_LIBCLANG_RECOGNIZES_FLAGS
 	select HAVE_SAMPLE_FTRACE_DIRECT
 	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
 	select HAVE_STACKPROTECTOR
@@ -236,6 +236,27 @@ config RUSTC_SUPPORTS_RISCV
 	# -Zsanitizer=shadow-call-stack flag.
 	depends on !SHADOW_CALL_STACK || RUSTC_VERSION >= 108200
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_FLAGS
+	def_bool y
+	depends on RUST_BINDGEN_LIBCLANG_RECOGNIZES_V
+	depends on RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZABHA
+	depends on RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZACAS
+	depends on RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZBA
+	depends on RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZBB
+	depends on RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZBC
+	depends on RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZBKB
+	depends on RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZICSR_ZIFENCEI
+	help
+	  Rust bindgen currently relies on libclang as backend. When a mixed build is
+	  performed (building C code with GCC), GCC flags will be passed to libclang.
+	  However, not all GCC flags are recognized by Clang, so most of the
+	  incompatible flags have been filtered out in rust/Makefile.
+
+	  For RISC-V, GCC and Clang are not at the same pace of implementing extensions.
+	  This config ensures that all important RISC-V specific flags that will be
+	  used by GCC can be correctly recognized by Rust bindgen's libclang, otherwise
+	  config HAVE_RUST will not be selected.
+
 config CLANG_SUPPORTS_DYNAMIC_FTRACE
 	def_bool CC_IS_CLANG
 	# https://github.com/ClangBuiltLinux/linux/issues/1817
@@ -634,6 +655,11 @@ config TOOLCHAIN_HAS_V
 	depends on LLD_VERSION >= 140000 || LD_VERSION >= 23800
 	depends on AS_HAS_OPTION_ARCH
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_V
+	def_bool y
+	# https://github.com/llvm/llvm-project/commit/e6de53b4de4aecca4ac892500a0907805896ed27
+	depends on !TOOLCHAIN_HAS_V || RUST_BINDGEN_LIBCLANG_VERSION >= 140000
+
 config RISCV_ISA_V
 	bool "Vector extension support"
 	depends on TOOLCHAIN_HAS_V
@@ -698,6 +724,11 @@ config TOOLCHAIN_HAS_ZABHA
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zabha)
 	depends on AS_HAS_OPTION_ARCH
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZABHA
+	def_bool y
+	# https://github.com/llvm/llvm-project/commit/6b7444964a8d028989beee554a1f5c61d16a1cac
+	depends on !TOOLCHAIN_HAS_ZABHA || RUST_BINDGEN_LIBCLANG_VERSION >= 190100
+
 config RISCV_ISA_ZABHA
 	bool "Zabha extension support for atomic byte/halfword operations"
 	depends on TOOLCHAIN_HAS_ZABHA
@@ -716,6 +747,11 @@ config TOOLCHAIN_HAS_ZACAS
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zacas)
 	depends on AS_HAS_OPTION_ARCH
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZACAS
+	def_bool y
+	# https://github.com/llvm/llvm-project/commit/614aeda93b2225c6eb42b00ba189ba7ca2585c60
+	depends on !TOOLCHAIN_HAS_ZACAS || RUST_BINDGEN_LIBCLANG_VERSION >= 200100
+
 config RISCV_ISA_ZACAS
 	bool "Zacas extension support for atomic CAS"
 	depends on TOOLCHAIN_HAS_ZACAS
@@ -735,6 +771,11 @@ config TOOLCHAIN_HAS_ZBB
 	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
 	depends on AS_HAS_OPTION_ARCH
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZBB
+	def_bool y
+	# https://github.com/llvm/llvm-project/commit/33d008b169f3c813a4a45da220d0952f795ac477
+	depends on !TOOLCHAIN_HAS_ZBB || RUST_BINDGEN_LIBCLANG_VERSION >= 140000
+
 # This symbol indicates that the toolchain supports all v1.0 vector crypto
 # extensions, including Zvk*, Zvbb, and Zvbc.  LLVM added all of these at once.
 # binutils added all except Zvkb, then added Zvkb.  So we just check for Zvkb.
@@ -750,6 +791,11 @@ config TOOLCHAIN_HAS_ZBA
 	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
 	depends on AS_HAS_OPTION_ARCH
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZBA
+	def_bool y
+	# https://github.com/llvm/llvm-project/commit/33d008b169f3c813a4a45da220d0952f795ac477
+	depends on !TOOLCHAIN_HAS_ZBA || RUST_BINDGEN_LIBCLANG_VERSION >= 140000
+
 config RISCV_ISA_ZBA
 	bool "Zba extension support for bit manipulation instructions"
 	default y
@@ -785,6 +831,11 @@ config TOOLCHAIN_HAS_ZBC
 	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
 	depends on AS_HAS_OPTION_ARCH
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZBC
+	def_bool y
+	# https://github.com/llvm/llvm-project/commit/33d008b169f3c813a4a45da220d0952f795ac477
+	depends on !TOOLCHAIN_HAS_ZBC || RUST_BINDGEN_LIBCLANG_VERSION >= 140000
+
 config RISCV_ISA_ZBC
 	bool "Zbc extension support for carry-less multiplication instructions"
 	depends on TOOLCHAIN_HAS_ZBC
@@ -808,6 +859,11 @@ config TOOLCHAIN_HAS_ZBKB
 	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
 	depends on AS_HAS_OPTION_ARCH
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZBKB
+	def_bool y
+	# https://github.com/llvm/llvm-project/commit/7ee1c162cc53d37f717f9a138276ad64fa6863bc
+	depends on !TOOLCHAIN_HAS_ZBKB || RUST_BINDGEN_LIBCLANG_VERSION >= 140000
+
 config RISCV_ISA_ZBKB
 	bool "Zbkb extension support for bit manipulation instructions"
 	depends on TOOLCHAIN_HAS_ZBKB
@@ -894,6 +950,10 @@ config TOOLCHAIN_NEEDS_OLD_ISA_SPEC
 	  versions of clang and GCC to be passed to GAS, which has the same result
 	  as passing zicsr and zifencei to -march.
 
+config RUST_BINDGEN_LIBCLANG_RECOGNIZES_ZICSR_ZIFENCEI
+	def_bool y
+	depends on TOOLCHAIN_NEEDS_OLD_ISA_SPEC || (TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI && RUST_BINDGEN_LIBCLANG_VERSION >= 170000)
+
 config FPU
 	bool "FPU support"
 	default y
diff --git a/rust/Makefile b/rust/Makefile
index 34d0429d50fd..7b1055c98146 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -277,20 +277,25 @@ bindgen_skip_c_flags := -mno-fp-ret-in-387 -mpreferred-stack-boundary=% \
 	-fno-inline-functions-called-once -fsanitize=bounds-strict \
 	-fstrict-flex-arrays=% -fmin-function-alignment=% \
 	-fzero-init-padding-bits=% -mno-fdpic \
-	--param=% --param asan-%
+	--param=% --param asan-% -mno-riscv-attribute
 
 # Derived from `scripts/Makefile.clang`.
 BINDGEN_TARGET_x86	:= x86_64-linux-gnu
 BINDGEN_TARGET_arm64	:= aarch64-linux-gnu
 BINDGEN_TARGET_arm	:= arm-linux-gnueabi
 BINDGEN_TARGET_loongarch	:= loongarch64-linux-gnusf
+BINDGEN_TARGET_riscv	:= riscv64-linux-gnu
 BINDGEN_TARGET_um	:= $(BINDGEN_TARGET_$(SUBARCH))
 BINDGEN_TARGET		:= $(BINDGEN_TARGET_$(SRCARCH))
 
+ifeq ($(BINDGEN_TARGET),)
+$(error add '--target=' option to rust/Makefile)
+else
 # All warnings are inhibited since GCC builds are very experimental,
 # many GCC warnings are not supported by Clang, they may only appear in
 # some configurations, with new GCC versions, etc.
 bindgen_extra_c_flags = -w --target=$(BINDGEN_TARGET)
+endif
 
 # Auto variable zero-initialization requires an additional special option with
 # clang that is going to be removed sometime in the future (likely in
-- 
2.51.0