Hi Marc,

On 1/27/21 2:35 PM, Marc Zyngier wrote:

Hi Alex,

On 2021-01-27 14:09, Alexandru Elisei wrote:

quoted

Hi Marc,

On 1/25/21 12:26 PM, Marc Zyngier wrote:

quoted

Upgrading the PMU code from ARMv8.1 to ARMv8.4 turns out to be
pretty easy. All that is required is support for PMMIR_EL1, which
is read-only, and for which returning 0 is a valid option as long
as we don't advertise STALL_SLOT as an implemented event.

According to ARM DDI 0487F.b, page D7-2743:

"If ARMv8.4-PMU is implemented:
- If STALL_SLOT is not implemented, it is IMPLEMENTATION DEFINED
whether the PMMIR
System registers are implemented.
- If STALL_SLOT is implemented, then the PMMIR System registers are
implemented."

I tried to come up with a reason why PMMIR is emulated instead of being left
undefined, but I couldn't figure it out. Would you mind adding a comment or
changing the commit message to explain that?

The main reason is that PMMIR gets new fields down the line,
and doing the bare minimum in term of implementation allows
us to gently ease into it.

I think I understand what you are saying - add a bare minimum emulation of the
PMMIR register now so it's less work when we do decide to support the STALL_SLOT
event for a guest.

We could also go for the full PMMIR reporting on homogeneous
systems too, as a further improvement.

What do you think?

I don't have an opinion either way. But if you do decide to add full emulation for
STALL_SLOT, I would like to help with reviewing the patches (I'm curious to see
how KVM will detect that it's running on a homogeneous system).

Thanks,
Alex

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