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
