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

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

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