[PATCH 4/4] s390: enable Rust support

[Jan Polensky]

Enable building Rust code on s390 by wiring the architecture into the
kernel Rust infrastructure.

Add s390 to the Rust arch support documentation, provide the s390 Rust
target and required compiler flags, and set the bindgen target for
arch/s390. Adjust the Rust target generation and minimum rustc version
gating so the s390 setup is handled explicitly.

The Rust toolchain uses the "s390x" triple naming for the 64 bit target.

Rust support is currently incompatible with CONFIG_EXPOLINE, which
relies on compiler support for the -mindirect-branch= and
-mfunction_return= options. Therefore, select HAVE_RUST only when
EXPOLINE is disabled.

Signed-off-by: Jan Polensky <japo@xxxxxxxxxxxxx>
---
 Documentation/rust/arch-support.rst |  1 +
 arch/s390/Kconfig                   |  1 +
 arch/s390/Makefile                  | 28 +++++++++++++++++-----------
 rust/Makefile                       |  1 +
 scripts/generate_rust_target.rs     |  2 ++
 scripts/min-tool-version.sh         |  6 +++++-
 6 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index 6e6a515d0899..ce9804b8226c 100644
--- a/Documentation/rust/arch-support.rst
+++ b/Documentation/rust/arch-support.rst
@@ -19,6 +19,7 @@ Architecture   Level of support  Constraints
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        \-
 ``riscv``      Maintained        ``riscv64`` and LLVM/Clang only.
+``s390``       Maintained        \-
 ``um``         Maintained        \-
 ``x86``        Maintained        ``x86_64`` only.
 =============  ================  ==============================================
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index ecbcbb781e40..26951781d74d 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -248,6 +248,7 @@ config S390
 	select HAVE_RELIABLE_STACKTRACE
 	select HAVE_RETHOOK
 	select HAVE_RSEQ
+	select HAVE_RUST if !EXPOLINE
 	select HAVE_SAMPLE_FTRACE_DIRECT
 	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
 	select HAVE_SETUP_PER_CPU_AREA
diff --git a/arch/s390/Makefile b/arch/s390/Makefile
index 297976b41088..c393c22c47c8 100644
--- a/arch/s390/Makefile
+++ b/arch/s390/Makefile
@@ -35,25 +35,31 @@ KBUILD_CFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO),-g)
 KBUILD_CFLAGS_DECOMPRESSOR += $(if $(CONFIG_DEBUG_INFO_DWARF4), $(call cc-option, -gdwarf-4,))
 KBUILD_CFLAGS_DECOMPRESSOR += $(if $(CONFIG_CC_NO_ARRAY_BOUNDS),-Wno-array-bounds)
 
+KBUILD_RUSTFLAGS += --target=s390x-unknown-none-softfloat -Zpacked-stack -Ctarget-feature=+backchain
+
 UTS_MACHINE	:= s390x
 STACK_SIZE	:= $(if $(CONFIG_KASAN),65536,$(if $(CONFIG_KMSAN),65536,16384))
 CHECKFLAGS	+= -D__s390__ -D__s390x__
 
 export LD_BFD
 
-mflags-$(CONFIG_MARCH_Z10)    := -march=z10
-mflags-$(CONFIG_MARCH_Z196)   := -march=z196
-mflags-$(CONFIG_MARCH_ZEC12)  := -march=zEC12
-mflags-$(CONFIG_MARCH_Z13)    := -march=z13
-mflags-$(CONFIG_MARCH_Z14)    := -march=z14
-mflags-$(CONFIG_MARCH_Z15)    := -march=z15
-mflags-$(CONFIG_MARCH_Z16)    := -march=z16
-mflags-$(CONFIG_MARCH_Z17)    := -march=z17
+march-name-$(CONFIG_MARCH_Z10)   := z10
+march-name-$(CONFIG_MARCH_Z196)  := z196
+march-name-$(CONFIG_MARCH_ZEC12) := zEC12
+march-name-$(CONFIG_MARCH_Z13)   := z13
+march-name-$(CONFIG_MARCH_Z14)   := z14
+march-name-$(CONFIG_MARCH_Z15)   := z15
+march-name-$(CONFIG_MARCH_Z16)   := z16
+march-name-$(CONFIG_MARCH_Z17)   := z17
 
-export CC_FLAGS_MARCH := $(mflags-y)
+mflags += -march=$(march-name-y)
 
-aflags-y += $(mflags-y)
-cflags-y += $(mflags-y)
+export CC_FLAGS_MARCH := $(mflags)
+
+aflags-y += $(mflags)
+cflags-y += $(mflags)
+
+KBUILD_RUSTFLAGS += -Ctarget-cpu=$(march-name-y)
 
 cflags-$(CONFIG_MARCH_Z10_TUNE)		+= -mtune=z10
 cflags-$(CONFIG_MARCH_Z196_TUNE)	+= -mtune=z196
diff --git a/rust/Makefile b/rust/Makefile
index b9e9f512cec3..77460502f576 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -403,6 +403,7 @@ BINDGEN_TARGET_x86	:= x86_64-linux-gnu
 BINDGEN_TARGET_arm64	:= aarch64-linux-gnu
 BINDGEN_TARGET_arm	:= arm-linux-gnueabi
 BINDGEN_TARGET_loongarch	:= loongarch64-linux-gnusf
+BINDGEN_TARGET_s390	:= s390x-linux-gnu
 # This is only for i386 UM builds, which need the 32-bit target not -m32
 BINDGEN_TARGET_i386	:= i386-linux-gnu
 BINDGEN_TARGET_um	:= $(BINDGEN_TARGET_$(SUBARCH))
diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
index 38b3416bb979..8f1df6819d0b 100644
--- a/scripts/generate_rust_target.rs
+++ b/scripts/generate_rust_target.rs
@@ -256,6 +256,8 @@ fn main() {
         }
     } else if cfg.has("LOONGARCH") {
         panic!("loongarch uses the builtin rustc loongarch64-unknown-none-softfloat target");
+    } else if cfg.has("S390") {
+        panic!("s390 uses the builtin rustc s390x-unknown-none-softfloat target");
     } else {
         panic!("Unsupported architecture");
     }
diff --git a/scripts/min-tool-version.sh b/scripts/min-tool-version.sh
index b96ec2d379b6..b1020cb9b755 100755
--- a/scripts/min-tool-version.sh
+++ b/scripts/min-tool-version.sh
@@ -31,7 +31,11 @@ llvm)
 	fi
 	;;
 rustc)
-	echo 1.85.0
+	if [ "$SRCARCH" = "s390" ]; then
+		echo 1.97.0-nightly
+	else
+		echo 1.85.0
+	fi
 	;;
 bindgen)
 	echo 0.71.1
-- 
2.51.0