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

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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 <omar.avelar@intel.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260916230003.1144622-1-chang.seok.bae@intel.com
This commit is contained in:
Chang S. Bae 2026-09-16 23:00:03 +00:00 committed by Borislav Petkov (AMD)
parent 96443a53bc
commit 63edf5a009
3 changed files with 21 additions and 0 deletions
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11
arch/x86/Kconfig.cpu
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@ -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

5
arch/x86/Makefile
View File

@ -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

5
scripts/generate_rust_target.rs
View File

@ -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"]);