[PATCH v3] x86/build/64: Prevent native builds from generating EGPR use

[Chang S. Bae]

Omar reported this broad concern to me, when resolving a separate issue
with his custom module. CONFIG_X86_NATIVE_CPU=y allows builds to
opportunistically emit instructions using %r16-%r31 (EGPRs) when the
build host supports APX since the commit:

  ea1dcca1de12 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")

But the kernel is not yet prepared to use new registers internally. For
example, there is no context-switch support for general in-kernel use.

Explicitly disable EGPR use when building with -march=native.

For C, since GCC 14 and Clang 18, both compilers support suppressing EGPR
use with -mno-apx-features=egpr, whose availability can be detected via
cc-option.

For Rust, pass features=-apxf through the generated JSON to avoid
unstable-feature warnings [1]. Note Rust only accepts the option to
disable APX instructions entirely or not.

Support for this gating also depends on the Rust/LLVM combination. Rust
1.88 introduced the `apxf` feature option, but versions prior to 1.93 may
emit an `apxf` attribute to the backend that only LLVM 23 or later can
interpret. Restrict native Rust builds accordingly.

Fixes: ea1dcca1de12 ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
Reported-by: Omar Avelar <omar.avelar@xxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Acked-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://github.com/rust-lang/rust/issues/139284 [1]
---
V2 -> V3:
 * Specify to avoid EGPR use for C compilers.
 * Add a link to track Rust APX support (Miguel Ojeda)
 * Clarify the commit in the changelog (Miguel Ojeda)
 * Then, collect Ojeda's Ack tag
 * Massage changelog and comments

Test for the C compiler sub-option:
 * Ensured apx-features= from the sample code [1], first
 * Earliest versions for the native builds were chosen to check EGPR emits.
 * Check EGPR strings appearance from kernel builds, like before [2].
   Nothing was found.
 * Note PUSH2 as new instructions are not much found, either. But
   regardless of the amount of APX bring-up to legacy GPR use,
   mno-apx-features=egpr seems to just be fine for avoiding EGPR use

[1] https://gcc.gnu.org/git/?p=gcc.git;a=tree;f=gcc/testsuite/gcc.target/i386
[2] https://lore.kernel.org/all/fbbe2b57-19de-4480-b0c6-f44a6007a38f@xxxxxxxxx/
---
 arch/x86/Kconfig.cpu            | 11 +++++++++++
 arch/x86/Makefile               |  5 +++++
 scripts/generate_rust_target.rs |  5 +++++
 3 files changed, 21 insertions(+)

diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
index e4654388d794..6e7a366f0798 100644
--- a/arch/x86/Kconfig.cpu
+++ b/arch/x86/Kconfig.cpu
@@ -204,10 +204,21 @@ config CC_HAS_MARCH_NATIVE
 	# usage warnings that only appear wth '-march=native'.
 	depends on CC_IS_GCC || CLANG_VERSION >= 190100
 
+config RUSTC_HAS_APXF
+	# The kernel isn't ready for in-kernel APX instructions. Without
+	# explicit frontend gating of APX, the backend may emit those
+	# instructions in native builds.
+	#
+	# Rust 1.88 added the `apxf` feature option, but versions before 1.93
+	# emit an `apxf` target attribute that only LLVM 23+ can interpret.
+	def_bool (RUSTC_VERSION >= 108800 && RUSTC_LLVM_MAJOR_VERSION >= 23) || \
+		 RUSTC_VERSION >= 109300
+
 config X86_NATIVE_CPU
 	bool "Build and optimize for local/native CPU"
 	depends on X86_64
 	depends on CC_HAS_MARCH_NATIVE
+	depends on !RUST || RUSTC_HAS_APXF
 	help
 	  Optimize for the current CPU used to compile the kernel.
 	  Use this option if you intend to build the kernel for your
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 598f178102ee..8af6b80cffdd 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -161,6 +161,11 @@ else
 
 ifdef CONFIG_X86_NATIVE_CPU
         KBUILD_CFLAGS += -march=native
+        # Prevent the compiler from generating EGPR use. The kernel is
+        # not yet prepared for general in-kernel use.
+        KBUILD_CFLAGS += $(call cc-option,-mno-apx-features=egpr)
+
+        # generate_rust_target.rs handles Rust APX gating.
         KBUILD_RUSTFLAGS += -Ctarget-cpu=native
 else
         KBUILD_CFLAGS += -march=x86-64 -mtune=generic
diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
index 3bf296581a88..7687b0dd5474 100644
--- a/scripts/generate_rust_target.rs
+++ b/scripts/generate_rust_target.rs
@@ -224,6 +224,11 @@ fn main() {
             features += ",+harden-sls-ijmp";
             features += ",+harden-sls-ret";
         }
+        if cfg.has("X86_NATIVE_CPU") {
+            // Prevent the backend from generating APX instructions. The kernel is not yet prepared
+            // for general in-kernel EGPR use.
+            features += ",-apxf";
+        }
         ts.push("features", features);
         ts.push("llvm-target", "x86_64-linux-gnu");
         ts.push("supported-sanitizers", ["kcfi", "kernel-address"]);
-- 
2.53.0