[PATCH v2 6/6] ARM: rust: Build Rust code for ARMv7 on ARMv7 kernels

[Karl Mehltretter]

The arm-unknown-linux-gnueabi target builds ARMv6 code. An ARMv7 kernel
gets that as well, because no CPU flags are passed to rustc. Its Rust
code cannot use movw/movt or any other ARMv7 instruction.

Use rustc's armv7-unknown-linux-gnueabi target when the kernel is built
with -march=armv7-a. It is ARMv7-A with the soft-float EABI and NEON
disabled. Unlike the generic target, it does not set +strict-align.
This allows unaligned word and halfword accesses, matching C code on
MMU-enabled ARMv7 kernels. A kernel that also contains an ARMv6K CPU
is built with -march=armv6k and keeps the generic target.

This builds the Rust code for the same architecture level as the C
code. In the tested Raspberry Pi 400 configuration, Rust text grows
by 1.9 %. Six small benchmarks range from 10 % shorter to 5 % longer
runtimes.

The generic target would turn a full memory barrier from
core::sync::atomic into the deprecated CP15 operation. The armv7 target
emits dmb. No kernel Rust code uses these atomics today, so this is not
a fix.

Tested on a Raspberry Pi 400 (Cortex-A72, LPAE) with clang 22.1.8 and
in QEMU virt (Cortex-A15) with the minimum tool versions. Rust modules
load with the movw/movt relocations.

Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
---
 arch/arm/Makefile | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index a72843890241..28eb8484ff82 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -73,9 +73,11 @@ arch-$(CONFIG_CPU_32v4T)	:=-march=armv4t
 arch-$(CONFIG_CPU_32v4)		:=-march=armv4
 arch-$(CONFIG_CPU_32v3)		:=-march=armv3m
 
-# The rustc target fixes the instruction set. -Ctarget-cpu cannot remove
-# the +v6 of the generic target, so older CPUs need their own target.
+# The rustc target fixes the instruction set, so pick it like -march above.
+# The generic target builds ARMv6 code. -Ctarget-cpu cannot remove its +v6.
 rust-target-y				:=arm-unknown-linux-gnueabi
+rust-target-$(CONFIG_CPU_32v7)		:=armv7-unknown-linux-gnueabi
+rust-target-$(CONFIG_CPU_32v6)		:=arm-unknown-linux-gnueabi
 rust-target-$(CONFIG_CPU_32v5)		:=armv5te-unknown-linux-gnueabi
 rust-target-$(CONFIG_CPU_32v4T)	:=armv4t-unknown-linux-gnueabi
 
-- 
2.39.5 (Apple Git-154)