If __riscv_hwprobe() fails, then the kernel version is presumably too
old. There is not much point falling back to the auxillary vector.
- The Linux kernel requires I, so the flag is always set on Linux, and
run-time detection is unnecessary. Our RISC-V assembler does anyway not
support targets without I.
- Linux can compile with or without F and D, but it cannot perform
run-time detection for them (a kernel with F support will not boot a
processor without F). The run-time detection is thus useless in that
case. Besides F and D extensions are used throughout the C code, so
their run-time detection would not be practical.
- Support for V was added in a later kernel version than riscv_hwprobe(),
so the system call will always be available if the kernel supports V.
The only exception would be vendor kernel forks, but those are known to
haphasardly pretend to support V on systems without actual V support, or
with only pre-ratification binary-incompatible version. Furthermore, a
large chunk of our optimisations require Zba and/or Zbb which cannot be
detected with HWCAP in those kernels.
For what it is worth, OpenJDK already took a similar action. Note that this
keeps AT_HWCAP usage for platforms with neither C run-time <sys/hwprobe.h>
nor kernel <asm/hwprobe.h>, notably kernels other than Linux.
Rémi Denis-Courmont