[PATCH] LoongArch: rust: Use rustc option -Zdirect-access-external-data

[PATCH] LoongArch: rust: Use rustc option -Zdirect-access-external-data

[WANG Rui]

-Zdirect-access-external-data is a new Rust compiler option added
in Rust 1.78, which we use to optimize the access of external data
in the Linux kernel's Rust code. This patch modifies the Rust code
in vmlinux to directly access externa data, using PC-REL instead of
GOT. However, Rust code whithin modules is constrained by the PC-REL
addressing range and is explicitly set to use an indirect method.

Signed-off-by: WANG Rui <wangrui@loongson.cn>
---
 arch/loongarch/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index 8674e7e24c4a..ae3f80622f4c 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -105,7 +105,8 @@ KBUILD_CFLAGS			+= -fno-jump-tables
 endif
 
 KBUILD_RUSTFLAGS		+= --target=loongarch64-unknown-none-softfloat
-KBUILD_RUSTFLAGS_MODULE		+= -Crelocation-model=pic
+KBUILD_RUSTFLAGS_KERNEL		+= -Zdirect-access-external-data=yes
+KBUILD_RUSTFLAGS_MODULE		+= -Zdirect-access-external-data=no
 
 ifeq ($(CONFIG_RELOCATABLE),y)
 KBUILD_CFLAGS_KERNEL		+= -fPIE
-- 
2.45.2

Re: [PATCH] LoongArch: rust: Use rustc option -Zdirect-access-external-data

[Miguel Ojeda]

On Wed, Jul 3, 2024 at 1:24 PM WANG Rui <wangrui@loongson.cn> wrote:
>
> -Zdirect-access-external-data is a new Rust compiler option added
> in Rust 1.78, which we use to optimize the access of external data
> in the Linux kernel's Rust code. This patch modifies the Rust code
> in vmlinux to directly access externa data, using PC-REL instead of
> GOT. However, Rust code whithin modules is constrained by the PC-REL
> addressing range and is explicitly set to use an indirect method.
>
> Signed-off-by: WANG Rui <wangrui@loongson.cn>

I have created a tracking issue for this in
https://github.com/rust-lang/rust/issues/127488 since there was only
the MCP, I hope that helps.

I also added it to our #2 list of unstable features -- please feel
free to ping me in the future if you add others that you will need in
the kernel, I am happy to add the entry even before you send the patch
to the kernel in order to track things (in fact, even if it is not
implemented we can still track it in the other lists).

Acked-by: Miguel Ojeda <ojeda@kernel.org>

Cheers,
Miguel