[PATCH v2 3/3] RISC-V: enable building 64-bit kernels with rust support

[Conor Dooley <conor@kernel.org>]

From: Miguel Ojeda <ojeda@kernel.org>

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,
taking the opportunity to sort the table in alphabetical order.

Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
---
 Documentation/rust/arch-support.rst | 1 +
 arch/riscv/Kconfig                  | 1 +
 arch/riscv/Makefile                 | 2 ++
 3 files changed, 4 insertions(+)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index 73203ba1e901..9e18a81fc2ef 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -16,6 +16,7 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 Architecture   Level of support  Constraints
 =============  ================  ==============================================
 ``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 5c59e00405e3..3eaae08e1d5c 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -142,6 +142,7 @@ 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_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HOTPLUG_CORE_SYNC_DEAD if HOTPLUG_CPU
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index ebbe02628a27..22fdb1e83744 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -34,6 +34,8 @@ ifeq ($(CONFIG_ARCH_RV64I),y)
 	KBUILD_AFLAGS += -mabi=lp64
 
 	KBUILD_LDFLAGS += -melf64lriscv
+
+	KBUILD_RUSTFLAGS += -Ctarget-cpu=generic-rv64
 else
 	BITS := 32
 	UTS_MACHINE := riscv32
-- 
2.43.0