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]
Chang S. Bae
63edf5a009