riscv: mm: Preserve global TLB entries when switching contexts

If the CPU does not support multiple ASIDs, all MM contexts use ASID 0.
In this case, it is still beneficial to flush the TLB by ASID, as the
single-ASID variant of the sfence.vma instruction preserves TLB entries
for global (kernel) pages.

This optimization is recommended by the RISC-V privileged specification:

  If the implementation does not provide ASIDs, or software chooses
  to always use ASID 0, then after every satp write, software should
  execute SFENCE.VMA with rs1=x0. In the common case that no global
  translations have been modified, rs2 should be set to a register
  other than x0 but which contains the value zero, so that global
  translations are not flushed.

It is not possible to apply this optimization when using the ASID
allocator, because that code must flush the TLB for all ASIDs at once
when incrementing the version number.

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20240327045035.368512-13-samuel.holland@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>