[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 <davi...@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

[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 <davi...@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

[Ard Biesheuvel]

Or simply

ifdef CONFIG_X86

[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