Linux kernel v6.16.2

The CONFIG_X86_NATIVE_CPU Flag:

- This is a CPU Virtualization error. Linux kernel 6.16+ versions introduce a new build time option, (CONFIG_X86_NATIVE_CPU) in the .config file, created after running _{make menuconfig}

When this option is enabled, i.e CONFIG_X86_NATIVE_CPU=y, then it instructs the compiler gcc to use the -march=native flag.

Now what does this flag mean or it's significance?

Essentially it directs the compiler to detect a specific CPU model on which the compiler runs and generate optimized instructions which will be unique to the architecture.

Now, on the host machine this option optimizes the code for the actual CPU but inside a VM (like VirtualBox), the compiler only sees the vCPU presented by the Virtual Environment which can be heavily restrictive and slightest modifications to the VM model via Host machine.

- The kernel which is built will have runtime portability issues and any changes to the VM physical configuration (via Host Machine Software) creates a fault wherein the kernel ELF file uses instructions that the new vCPU doesn't support!

Hence results in -> (Illegal Instruction Faults).

- Also with lower versions of GCC compiler, the flag -march=native is not supported completely (classic issue of legacy compatibility) and throws make errors - (build-time error).

Fix:

CONFIG_X86_NATIVE_CPU=y changed to CONFIG_X86_NATIVE_CPU=

Documentation on the merits/demerits of this feature: https://www.phoronix.com/review/linux-616-x86-native-cpu