[PATCH v5 4/4] RISC-V: handle extension configs for bindgen, re-enable gcc + rust builds

[Asuna Yang <spriteovo@gmail.com>]

From: 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. This commit enables GCC + Rust builds again.

Add `bindgen-option` conditions for the availability of libclang to the
RISC-V extension Kconfig symbols that depend on the `cc-option`
function.

For Zicsr/Zifencei special handling, since LLVM/Clang always enables
these two extensions, either don't pass them to `-march`, or pass them
explicitly and Rust bindgen libclang must recognize them.

Clang does not support `-mno-riscv-attribute` flag, filter it out to
resolve error: unknown argument: '-mno-riscv-attribute'.

Define `BINDGEN_TARGET_riscv` to pass the target triplet to Rust bindgen
libclang for RISC-V to resolve error: unsupported argument 'medany' to
option '-mcmodel=' for target 'unknown'.

Update the documentation, GCC + Rust builds for RISC-V are now
maintained.

Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Asuna Yang <SpriteOvO@gmail.com>
---
 Documentation/rust/arch-support.rst |  2 +-
 arch/riscv/Kconfig                  | 35 ++++++++++++++++++++++++++++++++++-
 rust/Makefile                       |  3 ++-
 scripts/Makefile.rust               |  1 +
 4 files changed, 38 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 fadec20b87a8..9a5606396646 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -193,7 +193,7 @@ 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 && TOOLCHAIN_MATCHES_ZICSR_ZIFENCEI
 	select HAVE_SAMPLE_FTRACE_DIRECT
 	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
 	select HAVE_STACKPROTECTOR
@@ -617,6 +617,8 @@ config TOOLCHAIN_HAS_V
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32imv)
 	depends on LD_IS_LLD || LD_VERSION >= 23800
 	depends on AS_HAS_OPTION_ARCH
+	depends on !RUST || !64BIT || $(bindgen-backend-option,-mabi=lp64 -march=rv64imv)
+	depends on !RUST || !32BIT || $(bindgen-backend-option,-mabi=ilp32 -march=rv32imv)
 
 config RISCV_ISA_V
 	bool "Vector extension support"
@@ -681,6 +683,8 @@ config TOOLCHAIN_HAS_ZABHA
 	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zabha)
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zabha)
 	depends on AS_HAS_OPTION_ARCH
+	depends on !RUST || !64BIT || $(bindgen-backend-option,-mabi=lp64 -march=rv64ima_zabha)
+	depends on !RUST || !32BIT || $(bindgen-backend-option,-mabi=ilp32 -march=rv32ima_zabha)
 
 config RISCV_ISA_ZABHA
 	bool "Zabha extension support for atomic byte/halfword operations"
@@ -699,6 +703,8 @@ config TOOLCHAIN_HAS_ZACAS
 	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zacas)
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zacas)
 	depends on AS_HAS_OPTION_ARCH
+	depends on !RUST || !64BIT || $(bindgen-backend-option,-mabi=lp64 -march=rv64ima_zacas)
+	depends on !RUST || !32BIT || $(bindgen-backend-option,-mabi=ilp32 -march=rv32ima_zacas)
 
 config RISCV_ISA_ZACAS
 	bool "Zacas extension support for atomic CAS"
@@ -717,6 +723,8 @@ config TOOLCHAIN_HAS_ZBB
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zbb)
 	depends on LD_IS_LLD || LD_VERSION >= 23900
 	depends on AS_HAS_OPTION_ARCH
+	depends on !RUST || !64BIT || $(bindgen-backend-option,-mabi=lp64 -march=rv64ima_zbb)
+	depends on !RUST || !32BIT || $(bindgen-backend-option,-mabi=ilp32 -march=rv32ima_zbb)
 
 # 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.
@@ -732,6 +740,8 @@ config TOOLCHAIN_HAS_ZBA
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zba)
 	depends on LD_IS_LLD || LD_VERSION >= 23900
 	depends on AS_HAS_OPTION_ARCH
+	depends on !RUST || !64BIT || $(bindgen-backend-option,-mabi=lp64 -march=rv64ima_zba)
+	depends on !RUST || !32BIT || $(bindgen-backend-option,-mabi=ilp32 -march=rv32ima_zba)
 
 config RISCV_ISA_ZBA
 	bool "Zba extension support for bit manipulation instructions"
@@ -767,6 +777,8 @@ config TOOLCHAIN_HAS_ZBC
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zbc)
 	depends on LD_IS_LLD || LD_VERSION >= 23900
 	depends on AS_HAS_OPTION_ARCH
+	depends on !RUST || !64BIT || $(bindgen-backend-option,-mabi=lp64 -march=rv64ima_zbc)
+	depends on !RUST || !32BIT || $(bindgen-backend-option,-mabi=ilp32 -march=rv32ima_zbc)
 
 config RISCV_ISA_ZBC
 	bool "Zbc extension support for carry-less multiplication instructions"
@@ -790,6 +802,8 @@ config TOOLCHAIN_HAS_ZBKB
 	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zbkb)
 	depends on LD_IS_LLD || LD_VERSION >= 23900
 	depends on AS_HAS_OPTION_ARCH
+	depends on !RUST || !64BIT || $(bindgen-backend-option,-mabi=lp64 -march=rv64ima_zbkb)
+	depends on !RUST || !32BIT || $(bindgen-backend-option,-mabi=ilp32 -march=rv32ima_zbkb)
 
 config RISCV_ISA_ZBKB
 	bool "Zbkb extension support for bit manipulation instructions"
@@ -877,6 +891,25 @@ 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_HAS_ZICSR_ZIFENCEI
+	def_bool y
+	depends on !64BIT || $(bindgen-backend-option,-mabi=lp64 -march=rv64ima_zicsr_zifencei)
+	depends on !32BIT || $(bindgen-backend-option,-mabi=ilp32 -march=rv32ima_zicsr_zifencei)
+
+config TOOLCHAIN_MATCHES_ZICSR_ZIFENCEI
+	def_bool y
+	# https://github.com/llvm/llvm-project/commit/22e199e6afb1263c943c0c0d4498694e15bf8a16
+	depends on TOOLCHAIN_NEEDS_OLD_ISA_SPEC || !TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI || RUST_BINDGEN_HAS_ZICSR_ZIFENCEI
+	help
+	  LLVM/Clang >= 17.0.0 starts recognizing Zicsr/Zifencei in -march, passing
+	  them to -march doesn't generate an error anymore, and passing them or not
+	  doesn't have any real difference, it still follows ISA before version
+	  20190608 - Zicsr/Zifencei are included in base ISA.
+
+	  The current latest version of LLVM/Clang still does not require explicit
+	  Zicsr/Zifencei to enable these two extensions, Clang just accepts them in
+	  -march and then silently ignores them.
+
 config FPU
 	bool "FPU support"
 	default y
diff --git a/rust/Makefile b/rust/Makefile
index 2603b34f9833..079af058ddde 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -383,7 +383,8 @@ 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-% -fno-isolate-erroneous-paths-dereference
+	--param=% --param asan-% -fno-isolate-erroneous-paths-dereference \
+	-mno-riscv-attribute
 
 # All warnings are inhibited since GCC builds are very experimental,
 # many GCC warnings are not supported by Clang, they may only appear in
diff --git a/scripts/Makefile.rust b/scripts/Makefile.rust
index b219244cd051..cee28a604830 100644
--- a/scripts/Makefile.rust
+++ b/scripts/Makefile.rust
@@ -3,6 +3,7 @@ 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))
 

-- 
2.51.1