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RISC-V Instruction Set Manual
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Unimplemented PMPs #1143

Open Timmmm opened 1 year ago

Timmmm commented 1 year ago

The spec says:

Up to 64 PMP entries are supported. Implementations may implement zero, 16, or 64 PMP entries; the lowest-numbered PMP entries must be implemented first. All PMP CSR fields are WARL and may be read-only zero.

This seems odd to me. Are you allowed to support e.g. 4 PMP entries by implementing 16 PMP registers but having 12 of them be read-only zero? If so why restrict the number of implemented PMP entries to 0, 16, 64?

A similar part of the spec is with HPM counters, but there the spec says that all HPM counters "should" be implemented, but can be read-only zero. Why not do the same for PMP entries?

allenjbaum commented 1 year ago

A fine question... which may become moot. Yes, if any PMP entries are implemented, then all 16 must be. Yes, they are allowed to be readOnly zero - but most importantly, must not trap. IF you implement 16, and try to read the 17th, then it will trap (at least now) unless all 64 are implemented. It would be legal to claim all 64 are implemented, even if only 4 are writable and the rest readOnly zero (or, actually, some other readOnly value, I believe).

I think the spec is being updated so that it will be legal to not trap if a CSR isn't implemented (much like executing an undefined op is not guaranteed to trap with an illegal op exception) though a profile could mandate that accesses to unimplemented CSR will trap.

On Mon, Oct 9, 2023 at 7:21 AM Tim Hutt @.***> wrote:

The spec says:

Up to 64 PMP entries are supported. Implementations may implement zero, 16, or 64 PMP entries; the lowest-numbered PMP entries must be implemented first. All PMP CSR fields are WARL and may be read-only zero.

This seems odd to me. Are you allowed to support e.g. 4 PMP entries by implementing 16 PMP registers but having 12 of them be read-only zero? If so why restrict the number of implemented PMP entries to 0, 16, 64?

A similar part of the spec is with HPM counters, but there the spec says that all HPM counters "should" be implemented, but can be read-only zero. Why not do the same for PMP entries?

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Omarezz123 commented 7 months ago

Spec not mention what if no pmp is implemented what will happen when accessing them will give illgeal read or make them read_zero (so not give illegal read)

gfavor commented 7 months ago

The answer is the same as for the CSRs of any extension that is not implemented. See chapter 2 of the latest integrated adoc version of the Priv spec.

Omarezz123 commented 7 months ago

can i have a link ?

gfavor commented 7 months ago

https://github.com/riscv/riscv-isa-manual

nick-knight commented 7 months ago

@Omarezz123

  1. Go here: https://github.com/riscv/riscv-isa-manual/releases (I have this in my browser's bookmark toolbar).
  2. Identify the latest release: it should be near the top of the page.
  3. Click "Assets" to expand a list of files. Volume I of the ISA specification is titled "unpriv-isa-asciidoc.pdf", and Volume II is "priv-isa-asciidoc.pdf". Volume II is the one Greg is pointing you to.