[PATCH v6 0/6] s390: Enable Rust support and add required arch glue

[PATCH v6 0/6] s390: Enable Rust support and add required arch glue

[Jan Polensky]

Rust support on s390 requires a small set of architecture-specific pieces
before the generic Rust kernel infrastructure can be used.

The series wires up s390 as a Rust-capable 64-bit architecture, adds the
missing assembly interfaces needed by Rust for WARN/BUG reporting and for
static branches, adjusts bindgen parameters to avoid repr layout conflicts
caused by packed and aligned s390 structures, and fixes issues discovered
during testing.

s390 currently requires rustc with support for -Zpacked-stack, and the
minimum tool version gating is adjusted accordingly.

Link: https://github.com/Rust-for-Linux/linux/issues/2

Tested against: rustc 1.96.0 (ac68faa20 2026-05-25)

Changes since v5:
- Fix indent in bug.h
- Add __rust_helper to rust_helper_memchr() (thanks Gary)

Changes since v4:
- Patch 4/6: Use kernel's memchr() instead of __builtin_memchr (Sashiko AI)

Changes since v3:
- Added patch 4/6: Add memchr wrapper preserving KASAN/FORTIFY_SOURCE (reported by Sashiko AI)
- Added patch 5/6: Fix KASAN stack-out-of-bounds in atomic helpers (reported by Miguel, suggested by Gary)
- Patch 6/6: Document CONFIG_EXPOLINE constraint in arch-support.rst (Sashiko AI feedback)
- Patch 6/6: Added Acked-by from Miguel Ojeda

Changes since v2:
- mflags: cleanup (thanks Gary)

Changes since v1:
- strip the -nightly suffix in min-tool-version.sh (thanks Miguel and Alice)
- ARCH_JUMP_TABLE_ENTRY() moved up to align comments properly (thanks Gary)
- removed MONCODE_BUG to prevent 0U in non-C context in assembler (noted by Sashiko - AI)
- prevent environment pollution by explicit initialization mflag := (noted by Sashiko - AI)

Jan Polensky (6):
  s390/bug: Provide ARCH_WARN_ASM for Rust WARN/BUG support
  s390/jump_label: Implement ARCH_STATIC_BRANCH_JUMP_ASM and
    ARCH_STATIC_BRANCH_ASM macros
  rust/bindgen_parameters: Mark s390 types as opaque to prevent repr
    conflicts
  rust: helpers: Add memchr wrapper for string operations
  s390/cmpxchg: Fix KASAN stack-out-of-bounds in atomic helpers
  s390: Enable Rust support

 Documentation/rust/arch-support.rst |  1 +
 arch/s390/Kconfig                   |  1 +
 arch/s390/Makefile                  | 28 ++++++++++++++----------
 arch/s390/include/asm/bug.h         | 12 +++++++++++
 arch/s390/include/asm/cmpxchg.h     |  8 +++----
 arch/s390/include/asm/jump_label.h  | 33 +++++++++++++++++------------
 rust/Makefile                       |  1 +
 rust/bindgen_parameters             |  7 ++++++
 rust/helpers/helpers.c              |  1 +
 rust/helpers/string.c               |  8 +++++++
 scripts/generate_rust_target.rs     |  2 ++
 scripts/min-tool-version.sh         |  6 +++++-
 12 files changed, 78 insertions(+), 30 deletions(-)
 create mode 100644 rust/helpers/string.c


base-commit: 2d3090a8aeb596a26935db0955d46c9a5db5c6ce
--
2.53.0

[PATCH v6 1/6] s390/bug: Provide ARCH_WARN_ASM for Rust WARN/BUG support

[Jan Polensky]

Rust WARN and BUG support relies on ARCH_WARN_ASM to emit __bug_table
entries. On s390 the macro is missing, so Rust code cannot generate
proper WARN/BUG metadata for the kernel's bug reporting infrastructure.

Define ARCH_WARN_ASM to produce the same assembly sequence and
__bug_table entry format as the existing s390 BUG handling, including
the monitor call. Define ARCH_WARN_REACHABLE as empty since s390 does
not provide reachability analysis for warning paths.

Acked-by: Gary Guo <gary@garyguo.net>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 arch/s390/include/asm/bug.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/s390/include/asm/bug.h b/arch/s390/include/asm/bug.h
index 50a270edb020..2010342c97b1 100644
--- a/arch/s390/include/asm/bug.h
+++ b/arch/s390/include/asm/bug.h
@@ -4,6 +4,7 @@
 
 #include <linux/compiler.h>
 #include <linux/const.h>
+#include <linux/stringify.h>
 
 #define	MONCODE_BUG	_AC(0, U)
 #define	MONCODE_BUG_ARG _AC(1, U)
@@ -121,6 +122,17 @@ do {									\
 #define HAVE_ARCH_BUG_FORMAT
 #define HAVE_ARCH_BUG_FORMAT_ARGS
 
+#define ARCH_WARN_ASM(file, line, flags, size)				\
+	".section .rodata.str,\"aMS\",@progbits,1\n"			\
+	"9:\n"								\
+	".asciz \"\"\n"		/* Empty string for compatibility */	\
+	".previous\n"							\
+	"0:\n"								\
+	__stringify(mc 0(%r0),0) "\n"					\
+	__BUG_ENTRY("9b", file, line, flags, size)
+
+#define ARCH_WARN_REACHABLE
+
 #endif /* CONFIG_BUG && CONFIG_CC_HAS_ASM_IMMEDIATE_STRINGS */
 #endif /* __ASSEMBLER__ */
 
-- 
2.53.0

[PATCH v6 2/6] s390/jump_label: Implement ARCH_STATIC_BRANCH_JUMP_ASM and ARCH_STATIC_BRANCH_ASM macros

[Jan Polensky]

Rust static branch support needs the s390 jump label instruction sequence
and __jump_table emission in a reusable form. The current implementation
embeds the sequence directly in the C asm goto blocks, which cannot be
shared with Rust.

Introduce ARCH_STATIC_BRANCH_ASM and ARCH_STATIC_BRANCH_JUMP_ASM to
describe the brcl sequences for the likely-false and likely-true cases
and to emit the same __jump_table entries as before. Switch the existing
C helpers to use the new macros to avoid duplication without changing
the generated code.

Acked-by: Gary Guo <gary@garyguo.net>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 arch/s390/include/asm/jump_label.h | 33 +++++++++++++++++-------------
 1 file changed, 19 insertions(+), 14 deletions(-)

diff --git a/arch/s390/include/asm/jump_label.h b/arch/s390/include/asm/jump_label.h
index d9cbc18f6b2e..0e28c90dc242 100644
--- a/arch/s390/include/asm/jump_label.h
+++ b/arch/s390/include/asm/jump_label.h
@@ -19,19 +19,29 @@
 #define JUMP_LABEL_STATIC_KEY_CONSTRAINT "jdd"
 #endif
 
+#define ARCH_JUMP_TABLE_ENTRY(key, label, local_label)	\
+	".pushsection __jump_table,\"aw\"\n"		\
+	".balign	8\n"				\
+	".long	" local_label "-.," label "-.\n"	\
+	".quad	" key "-.\n"				\
+	".popsection\n"
+
 /*
  * We use a brcl 0,<offset> instruction for jump labels so it
  * can be easily distinguished from a hotpatch generated instruction.
  */
+#define ARCH_STATIC_BRANCH_ASM(key, label)	\
+	"0:	brcl 0," label "\n"		\
+	ARCH_JUMP_TABLE_ENTRY(key, label, "0b")
+
+#define ARCH_STATIC_BRANCH_JUMP_ASM(key, label)	\
+	"0:	brcl 15," label "\n"		\
+	ARCH_JUMP_TABLE_ENTRY(key, label, "0b")
+
 static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
 {
-	asm goto("0:	brcl 0,%l[label]\n"
-			  ".pushsection __jump_table,\"aw\"\n"
-			  ".balign	8\n"
-			  ".long	0b-.,%l[label]-.\n"
-			  ".quad	%0+%1-.\n"
-			  ".popsection\n"
-			  : : JUMP_LABEL_STATIC_KEY_CONSTRAINT (key), "i" (branch) : : label);
+	asm goto(ARCH_STATIC_BRANCH_ASM("%0+%1", "%l[label]")
+		: : JUMP_LABEL_STATIC_KEY_CONSTRAINT (key), "i" (branch) : : label);
 	return false;
 label:
 	return true;
@@ -39,13 +49,8 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
 
 static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
 {
-	asm goto("0:	brcl 15,%l[label]\n"
-			  ".pushsection __jump_table,\"aw\"\n"
-			  ".balign	8\n"
-			  ".long	0b-.,%l[label]-.\n"
-			  ".quad	%0+%1-.\n"
-			  ".popsection\n"
-			  : : JUMP_LABEL_STATIC_KEY_CONSTRAINT (key), "i" (branch) : : label);
+	asm goto(ARCH_STATIC_BRANCH_JUMP_ASM("%0+%1", "%l[label]")
+		: : JUMP_LABEL_STATIC_KEY_CONSTRAINT (key), "i" (branch) : : label);
 	return false;
 label:
 	return true;
-- 
2.53.0

[PATCH v6 3/6] rust/bindgen_parameters: Mark s390 types as opaque to prevent repr conflicts

[Jan Polensky]

Bindgen attempts to generate Rust layouts for a number of s390 structs
that are packed but contain, or transitively contain, aligned fields.
Rust rejects such layouts with E0588 ("packed type cannot transitively
contain a #[repr(align)] type").

Add the affected s390 types to the opaque type list so bindgen emits
opaque blob types instead of full representations. This matches existing
workarounds for x86 types such as alt_instr and x86_msi_data.

Link: https://lore.kernel.org/all/e5c7aa10-590d-0d20-dd3b-385bee2377e7@intel.com/
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 rust/bindgen_parameters | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/rust/bindgen_parameters b/rust/bindgen_parameters
index 6f02d9720ad2..8402b0c93545 100644
--- a/rust/bindgen_parameters
+++ b/rust/bindgen_parameters
@@ -14,6 +14,13 @@
 --opaque-type alt_instr
 --opaque-type x86_msi_data
 --opaque-type x86_msi_addr_lo
+# s390-only: same packed/align issue as above (E0588).
+--opaque-type lowcore
+--opaque-type tod_clock
+--opaque-type tpi_info
+--opaque-type uv_cb.*
+--opaque-type uv_secret.*
+--opaque-type zpci_fib
 
 # If SMP is disabled, `arch_spinlock_t` is defined as a ZST which triggers a Rust
 # warning. We don't need to peek into it anyway.
-- 
2.53.0

[PATCH v6 4/6] rust: helpers: Add memchr wrapper for string operations

[Jan Polensky]

Add a dedicated string helper file with a memchr wrapper that uses the
kernel's instrumented memchr() function to ensure KASAN and FORTIFY_SOURCE
protections are preserved for Rust code.

Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Link: https://lore.kernel.org/rust-for-linux/CANiq72mXAZc0sNM7ShX8VDVs_7zJddawP-e=wt+ERr1YUCcWUw@mail.gmail.com/
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 rust/helpers/helpers.c | 1 +
 rust/helpers/string.c  | 8 ++++++++
 2 files changed, 9 insertions(+)
 create mode 100644 rust/helpers/string.c

diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 625921e27dfb..592b9bdb4e4a 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -88,6 +88,7 @@
 #include "signal.c"
 #include "slab.c"
 #include "spinlock.c"
+#include "string.c"
 #include "sync.c"
 #include "task.c"
 #include "time.c"
diff --git a/rust/helpers/string.c b/rust/helpers/string.c
new file mode 100644
index 000000000000..8ef30eb07a15
--- /dev/null
+++ b/rust/helpers/string.c
@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/string.h>
+
+__rust_helper void *rust_helper_memchr(const void *s, int c, size_t n)
+{
+	return memchr(s, c, n);
+}
-- 
2.53.0

[PATCH v6 5/6] s390/cmpxchg: Fix KASAN stack-out-of-bounds in atomic helpers

[Jan Polensky]

The __arch_cmpxchg1, __arch_cmpxchg2, __arch_xchg1, and __arch_xchg2
functions emulate 1-byte and 2-byte atomic operations using 4-byte
cmpxchg instructions, since s390 lacks native 1/2-byte cmpxchg support.

When KASAN is enabled, the READ_ONCE() operations in these functions
trigger stack-out-of-bounds warnings because they perform 4-byte reads
when only 1 or 2 bytes should be accessed.

Mark these functions as __no_sanitize_or_inline to prevent KASAN
instrumentation while maintaining correct functionality.

This resolves the following KASAN error during rust_atomics KUnit tests:

  BUG: KASAN: stack-out-of-bounds in rust_helper_atomic_i8_xchg+0xb2/0xc0
  Read of size 4 at addr 001bff7ffdbefcf0 by task kunit_try_catch/142

Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Link: https://lore.kernel.org/rust-for-linux/CANiq72m4GVWFYqnxNtCHTPu7XcGewHB5LNwOoayTfnXs9pPbNg@mail.gmail.com/
Suggested-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/rust-for-linux/DITFTAVVHTNQ.380OHUHGTOI6M@garyguo.net/
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 arch/s390/include/asm/cmpxchg.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/s390/include/asm/cmpxchg.h b/arch/s390/include/asm/cmpxchg.h
index 008357996262..e6ac55cf3c17 100644
--- a/arch/s390/include/asm/cmpxchg.h
+++ b/arch/s390/include/asm/cmpxchg.h
@@ -35,7 +35,7 @@ static __always_inline u64 __csg_asm(u64 ptr, u64 old, u64 new)
 	return old;
 }
 
-static inline u8 __arch_cmpxchg1(u64 ptr, u8 old, u8 new)
+static __no_sanitize_or_inline u8 __arch_cmpxchg1(u64 ptr, u8 old, u8 new)
 {
 	union {
 		u8 b[4];
@@ -58,7 +58,7 @@ static inline u8 __arch_cmpxchg1(u64 ptr, u8 old, u8 new)
 	return old;
 }
 
-static inline u16 __arch_cmpxchg2(u64 ptr, u16 old, u16 new)
+static __no_sanitize_or_inline u16 __arch_cmpxchg2(u64 ptr, u16 old, u16 new)
 {
 	union {
 		u16 b[2];
@@ -173,7 +173,7 @@ static __always_inline u64 __arch_cmpxchg(u64 ptr, u64 old, u64 new, int size)
 
 void __xchg_called_with_bad_pointer(void);
 
-static inline u8 __arch_xchg1(u64 ptr, u8 x)
+static __no_sanitize_or_inline u8 __arch_xchg1(u64 ptr, u8 x)
 {
 	int shift = (3 ^ (ptr & 3)) << 3;
 	u32 mask, old, new;
@@ -188,7 +188,7 @@ static inline u8 __arch_xchg1(u64 ptr, u8 x)
 	return old >> shift;
 }
 
-static inline u16 __arch_xchg2(u64 ptr, u16 x)
+static __no_sanitize_or_inline u16 __arch_xchg2(u64 ptr, u16 x)
 {
 	int shift = (2 ^ (ptr & 2)) << 3;
 	u32 mask, old, new;
-- 
2.53.0

[PATCH v6 6/6] 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.

Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 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..4f980815e92a 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        ``CONFIG_EXPOLINE`` must be disabled.
 ``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..8b712cd85fcd 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 16f7e855e012..3bf296581a88 100644
--- a/scripts/generate_rust_target.rs
+++ b/scripts/generate_rust_target.rs
@@ -260,6 +260,8 @@ 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..296acf8f71aa 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 1.96.0
+	else
+		echo 1.85.0
+	fi
 	;;
 bindgen)
 	echo 0.71.1
-- 
2.53.0

Re: [PATCH v6 6/6] s390: Enable Rust support

[Gary Guo]

On Mon Jun 8, 2026 at 7:14 PM BST, Jan Polensky wrote:
> 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.

Does `-Zretpoline-external-thunk`/`-Zretpoline` not work for s390? Rust will
throw a warning saying that it doesn't recognize this for the target, but it
looks like it does generate target features
+retpoline-external-thunk,+retpoline-indirect-branches,+retpoline-indirect-calls
in LLVM IR.

Best,
Gary

>
> Acked-by: Miguel Ojeda <ojeda@kernel.org>
> Acked-by: Heiko Carstens <hca@linux.ibm.com>
> Signed-off-by: Jan Polensky <japo@linux.ibm.com>
> ---
>  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..4f980815e92a 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        ``CONFIG_EXPOLINE`` must be disabled.
>  ``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..8b712cd85fcd 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 16f7e855e012..3bf296581a88 100644
> --- a/scripts/generate_rust_target.rs
> +++ b/scripts/generate_rust_target.rs
> @@ -260,6 +260,8 @@ 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..296acf8f71aa 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 1.96.0
> +	else
> +		echo 1.85.0
> +	fi
>  	;;
>  bindgen)
>  	echo 0.71.1

Re: [PATCH v6 6/6] s390: Enable Rust support

[Jan Polensky]

On Mon, Jun 08, 2026 at 07:25:06PM +0100, Gary Guo wrote:
> On Mon Jun 8, 2026 at 7:14 PM BST, Jan Polensky wrote:
> > 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.
>
> Does `-Zretpoline-external-thunk`/`-Zretpoline` not work for s390? Rust will
> throw a warning saying that it doesn't recognize this for the target, but it
> looks like it does generate target features
> +retpoline-external-thunk,+retpoline-indirect-branches,+retpoline-indirect-calls
> in LLVM IR.
>
> Best,
> Gary
>

Hi Gary,

thanks. The warning by itself does not prove the flags are ignored,
since rustc may still forward unknown target features to the backend.

That said, s390 currently has no Rust/LLVM support for the EXPOLINE
compiler support we rely on in C, and the kernel-side Rust retpoline
handling is only wired up for x86/x86_64 today.

Backend support is planned, but has not been implemented yet. Until then,
keeping Rust support gated behind !CONFIG_EXPOLINE is the intentional and
conservative choice for this series.

Best,
Jan

Re: [PATCH v6 6/6] s390: Enable Rust support

[Alexander Gordeev]

On Mon, Jun 08, 2026 at 09:45:41PM +0200, Jan Polensky wrote:

Hi Gary!

...

> Backend support is planned, but has not been implemented yet. Until then,
> keeping Rust support gated behind !CONFIG_EXPOLINE is the intentional and
> conservative choice for this series.

Is there anything in this series of concern to you?

Thanks!

> Best,
> Jan

Re: [PATCH v6 6/6] s390: Enable Rust support

[Gary Guo]

On Tue Jun 9, 2026 at 2:51 PM BST, Alexander Gordeev wrote:
> On Mon, Jun 08, 2026 at 09:45:41PM +0200, Jan Polensky wrote:
>
> Hi Gary!
>
> ...
>
>> Backend support is planned, but has not been implemented yet. Until then,
>> keeping Rust support gated behind !CONFIG_EXPOLINE is the intentional and
>> conservative choice for this series.
>
> Is there anything in this series of concern to you?
>
> Thanks!

It looks okay to me. Feel free to add my acks for patches that don't have any
yet.

Best,
Gary

Re: [PATCH v6 6/6] s390: Enable Rust support

[Nathan Chancellor]

Hi all,

On Mon, Jun 08, 2026 at 08:14:51PM +0200, Jan Polensky wrote:
> 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.
> 
> Acked-by: Miguel Ojeda <ojeda@kernel.org>
> Acked-by: Heiko Carstens <hca@linux.ibm.com>
> Signed-off-by: Jan Polensky <japo@linux.ibm.com>
...
> diff --git a/arch/s390/Makefile b/arch/s390/Makefile
> index 297976b41088..8b712cd85fcd 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

With Rust 1.96.0, I see the following warning several times when
building Rust code in -next:

  warning: unstable feature specified for `-Ctarget-feature`: `backchain`
    |
    = note: this feature is not stably supported; its behavior can change in the future

  warning: 1 warning emitted

I assume this is expected? If so, is there a way to silence this warning?

-- 
Cheers,
Nathan

Re: [PATCH v6 6/6] s390: Enable Rust support

[Miguel Ojeda]

On Mon, Jun 15, 2026 at 6:40 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> With Rust 1.96.0, I see the following warning several times when
> building Rust code in -next:
>
>   warning: unstable feature specified for `-Ctarget-feature`: `backchain`
>     |
>     = note: this feature is not stably supported; its behavior can change in the future
>
>   warning: 1 warning emitted
>
> I assume this is expected? If so, is there a way to silence this warning?

Yes, it seems so -- please see this thread:

  https://lore.kernel.org/rust-for-linux/CANiq72m4GVWFYqnxNtCHTPu7XcGewHB5LNwOoayTfnXs9pPbNg@mail.gmail.com/

If they are OK with it, then I guess it is fine, but it does mean CIs
can suffer a bit from it.

I hope that helps.

Cheers,
Miguel

Re: [PATCH v6 6/6] s390: Enable Rust support

[Nathan Chancellor]

On Mon, Jun 15, 2026 at 06:45:26PM +0200, Miguel Ojeda wrote:
> On Mon, Jun 15, 2026 at 6:40 PM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > With Rust 1.96.0, I see the following warning several times when
> > building Rust code in -next:
> >
> >   warning: unstable feature specified for `-Ctarget-feature`: `backchain`
> >     |
> >     = note: this feature is not stably supported; its behavior can change in the future
> >
> >   warning: 1 warning emitted
> >
> > I assume this is expected? If so, is there a way to silence this warning?
> 
> Yes, it seems so -- please see this thread:
> 
>   https://lore.kernel.org/rust-for-linux/CANiq72m4GVWFYqnxNtCHTPu7XcGewHB5LNwOoayTfnXs9pPbNg@mail.gmail.com/
> 
> If they are OK with it, then I guess it is fine, but it does mean CIs
> can suffer a bit from it.
> 
> I hope that helps.

Ah, thanks for the pointer and sorry for the duplicate report. I have
gone ahead and ignored this warning in my build reports for the time
being.

-- 
Cheers,
Nathan