x86/build/64: Prevent native builds from generating EGPR use

Omar reports that CONFIG_X86_NATIVE_CPU=y allows builds to opportunistically
emit instructions using %r16-%r31 (EGPRs) when the build host supports APX
since the commit:

  ea1dcca1de ("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, see

  https://github.com/rust-lang/rust/issues/139284

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: ea1dcca1de ("x86/kbuild/64: Add the CONFIG_X86_NATIVE_CPU option to locally optimize the kernel with '-march=native'")
Reported-by: Omar Avelar <[email protected]>
Signed-off-by: Chang S. Bae <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Reviewed-by: Nathan Chancellor <[email protected]>
Acked-by: Miguel Ojeda <[email protected]>
Cc: [email protected]
Link: https://patch.msgid.link/[email protected]

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

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

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"]);