About hpmevent platform-specific possibilities

[ThinkOpenly]

The spec seems to contradict itself, as I read it:

The Zihpm extension comprises up to 29 additional unprivileged 64-bit hardware performance counters, hpmcounter3-hpmcounter31. When XLEN=32, the upper 32 bits of these performance counters are accessible via additional CSRs hpmcounter3h- hpmcounter31h.
The implemented number and width of these additional counters, and the set of events they count, is platform-specific. Accessing an unimplemented or ill-configured counter may cause an illegal-instruction exception or may return a constant value.

The first paragraph says the hpmcounter* registers are 64-bits wide. The second paragraph says the widths are platform-specific.

If the width(s) are platform-specific, what are the possibilities? 32 and 64, or any width > 0?

Can different registers be different widths?

Are all the possible "constant values" that could be returned when accessing an implemented register identical?

What would be an example of an "ill-configured" register?

The simplest (and most logical) implementation I think would be:

• from 0 to 29 additional implemented registers
• all registers have identical widths
• valid widths are 32 or 64 bits
• if access to an unimplemented register returns a constant value, access to any unimplemented register returns the same constant value as any other unimplemented register

[aswaterman]

If you parse "implemented number and width" as "implemented number and implemented width", then it makes more sense. The appearance to software is that there's 64 bits of CSR, but not all of those bits might be implemented.

Many implementations have non-power-of-2 implemented counter widths; I recall having seen 40 and 48.

[ved-rivos]

Some counters may only count a fixed event and thus their event select may only allow selecting 0 and one other value. All counters may not have identical width - for instance the fixed counters may have a width sufficient for the rate of the event they count.

[ThinkOpenly]

Thanks for you responses. Let me make sure I understand.

The first paragraph says the hpmcounter* registers are 64-bits wide. The second paragraph says the widths are platform-specific.

If the width(s) are platform-specific, what are the possibilities? 32 and 64, or any width > 0?

Software sees all registers as present, but they may be read-only.
Software sees all registers as 64-bit.
The actual implementation may utilize fewer bits.

Can different registers be different widths?

Not from a software view, but it is possible that fewer bits are implemented in any register.

Are all the possible "constant values" that could be returned when accessing an implemented register identical?

I think this is "no"?

What would be an example of an "ill-configured" register?

I'm still not sure about the definition of "ill-configured". Is this the same as "not implemented"?