[PATCH v3 6/6] ARM: rust: Build Rust code for ARMv7 on ARMv7 kernels

[Karl Mehltretter <kmehltretter@gmail.com>]

The arm-unknown-linux-gnueabi target builds ARMv6 code, so ARMv7 kernels
cannot use ARMv7 instructions in Rust code.

Use armv7-unknown-linux-gnueabi when C is built with -march=armv7-a. The
target uses the soft-float EABI with NEON disabled and permits unaligned
word and halfword accesses, matching C on MMU-enabled ARMv7 kernels. A
kernel that also contains an ARMv6K CPU is built with -march=armv6k and
keeps the generic target.

In the tested Raspberry Pi 400 configuration, Rust text grows by 1.9%.
Six small benchmarks range from 10% shorter to 5% longer runtimes.

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@arndb.de>
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 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)