Using .arch here is not entirely future proof, but does work. The Linux kernel doesn't use .arch_extension. It figures out the highest configured -march= value, and for binutils it passes that to GNU as via -Wa,-march=<mumble> but for clang it uses .arch <mumble> via a special wrapper macro. That approach though spans the entire kernel build for GNU as, whereas this approach only affects pmap.c so will hopefully be easier to update if needed in the future.

For some related reading: https://www.sourceware.org/bugzilla/show_bug.cgi?id=26339

Why not use the new SYS_ARG macro? The tlbi instructions are in the sys instruction space.

I created macros in D59147 & D59148 to use the SYS_ARG macro

I'm curious as to why this isn't also a problem for the uses of .arch_extension in arm64/vfp.c, arm64/mte.c, and include/atomic.h?