arch: um: rust: Add i386 support for Rust

At present, Rust in the kernel only supports 64-bit x86, so UML has
followed suit. However, it's significantly easier to support 32-bit i386
on UML than on bare metal, as UML does not use the -mregparm option
(which alters the ABI), which is not yet supported by rustc[1].

Add support for CONFIG_RUST on um/i386, by adding a new target config to
generate_rust_target, and replacing various checks on CONFIG_X86_64 to
also support CONFIG_X86_32.

We still use generate_rust_target, rather than a built-in rustc target,
in order to match x86_64, provide a future place for -mregparm, and more
easily disable floating point instructions.

With these changes, the KUnit tests pass with:
kunit.py run --make_options LLVM=1 --kconfig_add CONFIG_RUST=y
--kconfig_add CONFIG_64BIT=n --kconfig_add CONFIG_FORTIFY_SOURCE=n

An earlier version of these changes was proposed on the Rust-for-Linux
github[2].

[1]: https://github.com/rust-lang/rust/issues/116972
[2]: https://github.com/Rust-for-Linux/linux/pull/966

Signed-off-by: David Gow <davidgow@google.com>
Link: https://patch.msgid.link/20240604224052.3138504-1-davidgow@google.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Documentation/rust/arch-support.rst

@@ -18,7 +18,7 @@ Architecture   Level of support  Constraints
 ``arm64``      Maintained        Little Endian only.
 ``loongarch``  Maintained        \-
 ``riscv``      Maintained        ``riscv64`` only.
-``um``         Maintained        ``x86_64`` only.
+``um``         Maintained        \-
 ``x86``        Maintained        ``x86_64`` only.
 =============  ================  ==============================================
 

arch/um/Kconfig

@@ -31,7 +31,7 @@ config UML
 	select TRACE_IRQFLAGS_SUPPORT
 	select TTY # Needed for line.c
 	select HAVE_ARCH_VMAP_STACK
-	select HAVE_RUST			if X86_64
+	select HAVE_RUST
 
 config MMU
 	bool

rust/Makefile

@@ -426,7 +426,7 @@ $(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--re
 $(obj)/core.o: private rustc_target_flags = $(core-cfgs)
 $(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
 	+$(call if_changed_dep,rustc_library)
-ifdef CONFIG_X86_64
+ifneq ($(or $(CONFIG_X86_64),$(CONFIG_X86_32)),)
 $(obj)/core.o: scripts/target.json
 endif
 

scripts/Makefile

@@ -12,7 +12,7 @@ hostprogs-always-$(CONFIG_SYSTEM_EXTRA_CERTIFICATE)	+= insert-sys-cert
 hostprogs-always-$(CONFIG_RUST_KERNEL_DOCTESTS)		+= rustdoc_test_builder
 hostprogs-always-$(CONFIG_RUST_KERNEL_DOCTESTS)		+= rustdoc_test_gen
 
-ifdef CONFIG_X86_64
+ifneq ($(or $(CONFIG_X86_64),$(CONFIG_X86_32)),)
 always-$(CONFIG_RUST)					+= target.json
 filechk_rust_target = $< < include/config/auto.conf
 

scripts/generate_rust_target.rs

@@ -169,6 +169,23 @@ fn main() {
         ts.push("features", features);
         ts.push("llvm-target", "x86_64-linux-gnu");
         ts.push("target-pointer-width", "64");
+    } else if cfg.has("X86_32") {
+        // This only works on UML, as i386 otherwise needs regparm support in rustc
+        if !cfg.has("UML") {
+            panic!("32-bit x86 only works under UML");
+        }
+        ts.push("arch", "x86");
+        ts.push(
+            "data-layout",
+            "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128",
+        );
+        let mut features = "-3dnow,-3dnowa,-mmx,+soft-float".to_string();
+        if cfg.has("MITIGATION_RETPOLINE") {
+            features += ",+retpoline-external-thunk";
+        }
+        ts.push("features", features);
+        ts.push("llvm-target", "i386-unknown-linux-gnu");
+        ts.push("target-pointer-width", "32");
     } else if cfg.has("LOONGARCH") {
         panic!("loongarch uses the builtin rustc loongarch64-unknown-none-softfloat target");
     } else {