[RFC 2/2] RISC-V: enable building the 64-bit kernels with rust support

[Conor Dooley]

From: Miguel Ojeda <ojeda at kernel.org>

The rust modules work on 64-bit RISC-V, with no twiddling required.
Select HAS_RUST and provide the required flags to kbuild so that the
modules can be used.
32-bit is broken in core rust code, so support is limited to 64-bit
only: ld.lld: error: undefined symbol: __udivdi3

Signed-off-by: Miguel Ojeda <ojeda at kernel.org>
Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
---
 Documentation/rust/arch-support.rst | 2 ++
 arch/riscv/Kconfig                  | 1 +
 arch/riscv/Makefile                 | 3 ++-
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index 6982b63775da..197919158596 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -15,5 +15,7 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 ============  ================  ==============================================
 Architecture  Level of support  Constraints
 ============  ================  ==============================================
+``riscv``     Maintained        ``rv64`` only.
+============  ================  ==============================================
 ``x86``       Maintained        ``x86_64`` only.
 ============  ================  ==============================================
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 81eb031887d2..73174157212d 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -115,6 +115,7 @@ config RISCV
 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RSEQ
+	select HAVE_RUST if 64BIT
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select IRQ_DOMAIN
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 76989561566b..f8b3f58f2e40 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -29,7 +29,7 @@ ifeq ($(CONFIG_ARCH_RV64I),y)
 
 	KBUILD_CFLAGS += -mabi=lp64
 	KBUILD_AFLAGS += -mabi=lp64
-
+	KBUILD_RUSTFLAGS += -Ctarget-cpu=generic-rv64
 	KBUILD_LDFLAGS += -melf64lriscv
 else
 	BITS := 32
@@ -38,6 +38,7 @@ else
 	KBUILD_CFLAGS += -mabi=ilp32
 	KBUILD_AFLAGS += -mabi=ilp32
 	KBUILD_LDFLAGS += -melf32lriscv
+	KBUILD_RUSTFLAGS += -Ctarget-cpu=generic-rv32
 endif
 
 ifeq ($(CONFIG_LD_IS_LLD),y)
-- 
2.39.2