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Since this was a bit hard to google:

ASID = Address Space Identifier. It's a tag that uniquely identifies each processes' entries in the TLB. This ensures that your TLB lookups can be limited to the valid entries for the process, so you don't need to flush the TLB on context switch.

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I think the way to think of ASIDs is as each being a separate address space - in effect if you have 15 bits of ASID you have 2^15 - 32k address spaces.

One thing I've done in VRoom! which is an extension on to the RISCV spec is that if we have an N hart SMP CPU (for example a 2 cpu SMT system) we use log2(N) bits of the ASID to select which hart/cpu a TLB entry belongs to - from a programmer's point of view the ASID just looks smaller.

However there's a VRoom! specific config bit (by default off) that you can set if you know that the ASIDs you are going to use for all your CPU's effectively see the same address space - if you set that bit then the per-cpu portion of the ASID tags (in the TLB) become available (ie to the programmer the ASID looks bigger) - it's a great hack because it doesn't get into any critical paths anywhere

https://github.com/riscv/riscv-isa-manual/issues/348 suggests that the RISC-V folks are going to address this in the spec at some point.

RISC-V really is the heir of MIPS as it had those TLB tags as well.

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