Re: [PATCH v2 1/3] arm64: rust: Enable Rust support for AArch64

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Jun 06, 2023 at 03:56:04PM +0100, Jamie Cunliffe wrote:
> This commit provides the build flags for Rust for AArch64. The core Rust
> support already in the kernel does the rest.
> 
> When disabling the neon and fp target features to avoid fp & simd
> registers. The use of fp-armv8 will cause a warning from rustc about
> an unknown feature that is specified. The target feature is still
> passed through to LLVM, this behaviour is documented as part of the
> warning. This will be fixed in a future version of the rustc
> toolchain.
> 
> The Rust samples have been tested with this commit.
> 
> Signed-off-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>
> ---
>  Documentation/rust/arch-support.rst | 1 +
>  Makefile                            | 1 -
>  arch/arm64/Kconfig                  | 1 +
>  arch/arm64/Makefile                 | 2 ++
>  arch/x86/Makefile                   | 1 +
>  rust/Makefile                       | 6 +++++-
>  scripts/Makefile                    | 5 +++--
>  scripts/generate_rust_target.rs     | 4 +++-
>  8 files changed, 16 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
> index b91e9ef4d0c2..9b022af2f649 100644
> --- a/Documentation/rust/arch-support.rst
> +++ b/Documentation/rust/arch-support.rst
> @@ -15,6 +15,7 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
>  ============  ================  ==============================================
>  Architecture  Level of support  Constraints
>  ============  ================  ==============================================
> +``arm64``     Maintained        None.
>  ``um``        Maintained        ``x86_64`` only.
>  ``x86``       Maintained        ``x86_64`` only.
>  ============  ================  ==============================================
> diff --git a/Makefile b/Makefile
> index 9d765ebcccf1..16b1e4de70e3 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -561,7 +561,6 @@ KBUILD_CFLAGS   := -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs \
>  		   -std=gnu11
>  KBUILD_CPPFLAGS := -D__KERNEL__
>  KBUILD_RUSTFLAGS := $(rust_common_flags) \
> -		    --target=$(objtree)/scripts/target.json \
>  		    -Cpanic=abort -Cembed-bitcode=n -Clto=n \
>  		    -Cforce-unwind-tables=n -Ccodegen-units=1 \
>  		    -Csymbol-mangling-version=v0 \
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b1201d25a8a4..10e9399e8e2f 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -217,6 +217,7 @@ config ARM64
>  	select HAVE_FUNCTION_ARG_ACCESS_API
>  	select MMU_GATHER_RCU_TABLE_FREE
>  	select HAVE_RSEQ
> +	select HAVE_RUST
>  	select HAVE_STACKPROTECTOR
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_KPROBES
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 2d49aea0ff67..2ce1555e9fc5 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -41,6 +41,8 @@ KBUILD_CFLAGS	+= -mgeneral-regs-only	\
>  KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
>  KBUILD_AFLAGS	+= $(compat_vdso)
>  
> +KBUILD_RUSTFLAGS += --target aarch64-unknown-none -C target-feature="-neon,-fp-armv8"

I don't think you need "-fp-armv8" here, because "aarch64-unknown-none"
target doesn't have the target-feature "fp-arm64":

	rustc +1.68.2 --print cfg --target aarch64-unknown-none | grep target_feature

	target_feature="neon"

, and having the "-fp-armv8" gave me a warning at compile time:

	  RUSTC L rust/core.o warning: unknown feature specified for `-Ctarget-feature`: `fp-armv8`
	  |
	  = note: it is still passed through to the codegen backend
	  = help: consider filing a feature request

Regards,
Boqun

> +
>  KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
>  KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)
>  
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index b39975977c03..79cffe92c916 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -68,6 +68,7 @@ export BITS
>  #    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
>  #
>  KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx
> +KBUILD_RUSTFLAGS += --target=$(objtree)/scripts/target.json
>  KBUILD_RUSTFLAGS += -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2
>  
>  ifeq ($(CONFIG_X86_KERNEL_IBT),y)
> diff --git a/rust/Makefile b/rust/Makefile
> index 7c9d9f11aec5..18d6fdb73fa9 100644
> --- a/rust/Makefile
> +++ b/rust/Makefile
> @@ -262,6 +262,7 @@ bindgen_skip_c_flags := -mno-fp-ret-in-387 -mpreferred-stack-boundary=% \
>  
>  # Derived from `scripts/Makefile.clang`.
>  BINDGEN_TARGET_x86	:= x86_64-linux-gnu
> +BINDGEN_TARGET_arm64    := aarch64-linux-gnu
>  BINDGEN_TARGET		:= $(BINDGEN_TARGET_$(SRCARCH))
>  
>  # All warnings are inhibited since GCC builds are very experimental,
> @@ -393,8 +394,11 @@ $(obj)/core.o: private skip_clippy = 1
>  $(obj)/core.o: private skip_flags = -Dunreachable_pub
>  $(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--redefine-sym $(sym)=__rust$(sym))
>  $(obj)/core.o: private rustc_target_flags = $(core-cfgs)
> -$(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs scripts/target.json FORCE
> +$(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
>  	$(call if_changed_dep,rustc_library)
> +ifeq ($(ARCH),x86_64)
> +$(obj)/core.o: scripts/target.json
> +endif
>  
>  $(obj)/compiler_builtins.o: private rustc_objcopy = -w -W '__*'
>  $(obj)/compiler_builtins.o: $(src)/compiler_builtins.rs $(obj)/core.o FORCE
> diff --git a/scripts/Makefile b/scripts/Makefile
> index 32b6ba722728..633479d02e33 100644
> --- a/scripts/Makefile
> +++ b/scripts/Makefile
> @@ -9,12 +9,13 @@ hostprogs-always-$(CONFIG_BUILDTIME_TABLE_SORT)		+= sorttable
>  hostprogs-always-$(CONFIG_ASN1)				+= asn1_compiler
>  hostprogs-always-$(CONFIG_MODULE_SIG_FORMAT)		+= sign-file
>  hostprogs-always-$(CONFIG_SYSTEM_EXTRA_CERTIFICATE)	+= insert-sys-cert
> -always-$(CONFIG_RUST)					+= target.json
>  
> +ifeq ($(ARCH),x86_64)
> +always-$(CONFIG_RUST)					+= target.json
>  filechk_rust_target = $< < include/config/auto.conf
> -
>  $(obj)/target.json: scripts/generate_rust_target include/config/auto.conf FORCE
>  	$(call filechk,rust_target)
> +endif
>  
>  hostprogs += generate_rust_target
>  generate_rust_target-rust := y
> diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
> index 3c6cbe2b278d..ec5ef35dbe52 100644
> --- a/scripts/generate_rust_target.rs
> +++ b/scripts/generate_rust_target.rs
> @@ -148,7 +148,9 @@ fn main() {
>      let mut ts = TargetSpec::new();
>  
>      // `llvm-target`s are taken from `scripts/Makefile.clang`.
> -    if cfg.has("X86_64") {
> +    if cfg.has("ARM64") {
> +        panic!("arm64 uses the builtin rustc aarch64-unknown-none target");
> +    } else if cfg.has("X86_64") {
>          ts.push("arch", "x86_64");
>          ts.push(
>              "data-layout",
> 
> -- 
> 2.30.2
>