[PATCH] arch: um: rust: Add i386 support for Rust

[PATCH] arch: um: rust: Add i386 support for Rust

[David Gow]

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>
---
 Documentation/rust/arch-support.rst |  2 +-
 arch/um/Kconfig                     |  2 +-
 rust/Makefile                       |  2 +-
 scripts/Makefile                    |  2 +-
 scripts/generate_rust_target.rs     | 17 +++++++++++++++++
 5 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
index b13e19d84744..750ff371570a 100644
--- a/Documentation/rust/arch-support.rst
+++ b/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.
 =============  ================  ==============================================
 
diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index 93a5a8999b07..b6ac49fec5bb 100644
--- a/arch/um/Kconfig
+++ b/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
diff --git a/rust/Makefile b/rust/Makefile
index f70d5e244fee..83f675adbfab 100644
--- a/rust/Makefile
+++ b/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
 
diff --git a/scripts/Makefile b/scripts/Makefile
index fe56eeef09dd..dccef663ca82 100644
--- a/scripts/Makefile
+++ b/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
 
diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
index 641b713a033a..87f34925eb7b 100644
--- a/scripts/generate_rust_target.rs
+++ b/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 {
-- 
2.45.1.288.g0e0cd299f1-goog

Re: [PATCH] arch: um: rust: Add i386 support for Rust

[Nathan Chancellor]

Hi David,

Just a fly by comment on style, I don't have much to say content :)

On Wed, Jun 05, 2024 at 06:40:50AM +0800, David Gow wrote:
> 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 probably like  support CONFIG_X86_32.
> 
> We still use generate_rust_target, rather than a built-in rustc target,
> in oPrder 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>
...
> -ifdef CONFIG_X86_64
> +ifneq ($(or $(CONFIG_X86_64),$(CONFIG_X86_32)),)

These configurations are mutually exclusive, so would it look more
readable to have it be:


ifeq ($(CONFIG_X86_32)$(CONFIG_X86_64),y)

?

Cheers.
Nathan

Re: [PATCH] arch: um: rust: Add i386 support for Rust

[Ard Biesheuvel]

On Wed, 5 Jun 2024 at 08:22, Nathan Chancellor <nathan@kernel.org> wrote:
>
> Hi David,
>
> Just a fly by comment on style, I don't have much to say content :)
>
> On Wed, Jun 05, 2024 at 06:40:50AM +0800, David Gow wrote:
> > 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 probably like  support CONFIG_X86_32.
> >
> > We still use generate_rust_target, rather than a built-in rustc target,
> > in oPrder 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>
> ...
> > -ifdef CONFIG_X86_64
> > +ifneq ($(or $(CONFIG_X86_64),$(CONFIG_X86_32)),)
>
> These configurations are mutually exclusive, so would it look more
> readable to have it be:
>
>
> ifeq ($(CONFIG_X86_32)$(CONFIG_X86_64),y)
>

Or simply

ifdef CONFIG_X86

Re: [PATCH] arch: um: rust: Add i386 support for Rust

[Johannes Berg]

On Wed, 2024-06-05 at 19:59 +0200, Ard Biesheuvel wrote:
> 
> > > -ifdef CONFIG_X86_64
> > > +ifneq ($(or $(CONFIG_X86_64),$(CONFIG_X86_32)),)
> > 
> > These configurations are mutually exclusive, so would it look more
> > readable to have it be:
> > 
> > 
> > ifeq ($(CONFIG_X86_32)$(CONFIG_X86_64),y)
> > 
> 
> Or simply
> 
> ifdef CONFIG_X86

No, we're talking about ARCH=um, so CONFIG_X86 isn't set, but
CONFIG_X86_{32,64} are set as the "sub" or "host" arch. :)

johannes