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

[Nathan Chancellor]

Hi Conor,

On Fri, Feb 24, 2023 at 01:50:44PM +0000, Conor Dooley wrote:
> 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

Just a small drive by comment, you have 'if 64BIT' here...

>  	select HAVE_STACKPROTECTOR
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select IRQ_DOMAIN
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 76989561566b..0d6fc4e25221 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -29,8 +29,8 @@ ifeq ($(CONFIG_ARCH_RV64I),y)
>  
>  	KBUILD_CFLAGS += -mabi=lp64
>  	KBUILD_AFLAGS += -mabi=lp64
> -
>  	KBUILD_LDFLAGS += -melf64lriscv
> +	KBUILD_RUSTFLAGS += -Ctarget-cpu=generic-rv64
>  else
>  	BITS := 32
>  	UTS_MACHINE := riscv32
> @@ -38,6 +38,7 @@ else
>  	KBUILD_CFLAGS += -mabi=ilp32
>  	KBUILD_AFLAGS += -mabi=ilp32
>  	KBUILD_LDFLAGS += -melf32lriscv
> +	KBUILD_RUSTFLAGS += -Ctarget-cpu=generic-rv32

but also add KBUILD_RUSTFLAGS for the !64BIT case. Seems like one of
those can be removed.

>  endif
>  
>  ifeq ($(CONFIG_LD_IS_LLD),y)
> -- 
> 2.39.2
> 

Cheers,
Nathan