[PATCH v2 6/7] KVM: arm64: Upgrade PMU support to ARMv8.4

[Marc Zyngier]

Hi Alex,

On 2021-01-27 14:09, Alexandru Elisei wrote:
> Hi Marc,
> 
> On 1/25/21 12:26 PM, Marc Zyngier wrote:
>> 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.

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

What do you think?

         M.
-- 
Jazz is not dead. It just smells funny...