[PATCH v3 1/1] RISC-V: enable building 64-bit kernels with rust support

[Conor Dooley]

From: Miguel Ojeda <ojeda@xxxxxxxxxx>

The rust modules work on 64-bit RISC-V, with no twiddling required.
Select HAVE_RUST and provide the required flags to kbuild so that the
modules can be used. The Makefile and Kconfig changes are lifted from
work done by Miguel in the Rust-for-Linux tree, hence his authorship.
Following the rabbit hole, the Makefile changes originated in a script,
created based on config files originally added by Gary, hence his
co-authorship.

32-bit is broken in core rust code, so support is limited to 64-bit:
ld.lld: error: undefined symbol: __udivdi3

As 64-bit RISC-V is now supported, add it to the arch support table.

Co-developed-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Co-developed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
Signed-off-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
---
 Documentation/rust/arch-support.rst | 1 +
 arch/riscv/Kconfig                  | 1 +
 arch/riscv/Makefile                 | 7 +++++++
 scripts/generate_rust_target.rs     | 6 ++++++
 4 files changed, 15 insertions(+)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index 5c4fa9f5d1cd..4d1495ded2aa 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -17,6 +17,7 @@ Architecture   Level of support  Constraints
 =============  ================  ==============================================
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        -
+``riscv``      Maintained        ``riscv64`` only.
 ``um``         Maintained        ``x86_64`` only.
 ``x86``        Maintained        ``x86_64`` only.
 =============  ================  ==============================================
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index be09c8836d56..cad31864fd0f 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -155,6 +155,7 @@ config RISCV
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RETHOOK if !XIP_KERNEL
 	select HAVE_RSEQ
+	select HAVE_RUST if 64BIT
 	select HAVE_SAMPLE_FTRACE_DIRECT
 	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
 	select HAVE_STACKPROTECTOR
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 252d63942f34..adbc9023d7f0 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -34,6 +34,9 @@ ifeq ($(CONFIG_ARCH_RV64I),y)
 	KBUILD_AFLAGS += -mabi=lp64
 
 	KBUILD_LDFLAGS += -melf64lriscv
+
+	KBUILD_RUSTFLAGS += -Ctarget-cpu=generic-rv64 --target=riscv64imac-unknown-none-elf \
+			    -Cno-redzone
 else
 	BITS := 32
 	UTS_MACHINE := riscv32
@@ -68,6 +71,10 @@ riscv-march-$(CONFIG_FPU)		:= $(riscv-march-y)fd
 riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
 riscv-march-$(CONFIG_RISCV_ISA_V)	:= $(riscv-march-y)v
 
+ifneq ($(CONFIG_RISCV_ISA_C),y)
+	KBUILD_RUSTFLAGS += -Ctarget-feature=-c
+endif
+
 ifdef CONFIG_TOOLCHAIN_NEEDS_OLD_ISA_SPEC
 KBUILD_CFLAGS += -Wa,-misa-spec=2.2
 KBUILD_AFLAGS += -Wa,-misa-spec=2.2
diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
index 54919cf48621..8f7846b9029a 100644
--- a/scripts/generate_rust_target.rs
+++ b/scripts/generate_rust_target.rs
@@ -150,6 +150,12 @@ fn main() {
     // `llvm-target`s are taken from `scripts/Makefile.clang`.
     if cfg.has("ARM64") {
         panic!("arm64 uses the builtin rustc aarch64-unknown-none target");
+    } else if cfg.has("RISCV") {
+        if cfg.has("64BIT") {
+            panic!("64-bit RISC-V uses the builtin rustc riscv64-unknown-none-elf target");
+        } else {
+            panic!("32-bit RISC-V is an unsupported architecture");
+        }
     } else if cfg.has("X86_64") {
         ts.push("arch", "x86_64");
         ts.push(
-- 
2.43.0