Page sizes supported by RISC-V

[Florian Weimer]

I'm working on adding a <sys/pagesize.h> header to glibc, so that
programmers can easily determine the range of possible page sizes for a
particular target, for things like pointer tagging and mapped file
alignment requirements.

Linux currently supports 4 KiB pages only.  I couldn't find anything in
the RISC-V specifications that mandates a specific page size, but of
course the way ELF LOAD segments are constructed necessarily imposes a
maximum page size constraint for particular executables.  Today, the
toolchain assumes the maximum supported page size is 4 KiB, and this is
also reflected in arch/riscv/Kconfig in the Linux sources (as far as I
can read Kconfig).

I found a previous proposal for kernel support of 64 KiB pages:

  [RFC PATCH V1 00/11] riscv: Introduce 64K base page
  <https://lore.kernel.org/linux-riscv/20231123065708.91345-1-luxu.kernel@bytedance.com/>

Is this something we should consider today?

I'm leaning towards teaching glibc that the RISC-V page size is always
4 KiB because given the current toolchain defaults, it's necessary
to rebuild the world anyway if supported page sizes ever change.

Thanks,
Florian