On Sun, Jun 11, 2023 at 09:01:05AM -0700, Reiji Watanabe wrote: [...] > > Suppose KVM is running on a v3p5+ implementation, but userspace has set > > ID_AA64DFR0_EL1.PMUVer to v3p0. In this case the read of PMCEID1_EL0 on > > the preceding line would advertise the STALL_SLOT event, and KVM fails > > to mask it due to the ID register value. The fact we do not support the > > event is an invariant, in the worst case we wind up clearing a bit > > that's already 0. > > As far as I checked ArmARM, the STALL_SLOT event can be supported on > any PMUv3 version (including on v3p0). Assuming that is true, I don't > see any reason to not expose the event to the guest in this particular > example. Or can the STALL_SLOT event only be implemented from certain > versions of PMUv3 ? Well, users of the event don't get the full picture w/o PMMIR_EL1.SLOTS, which is only available on v3p4+. We probably should start exposing the register + event (separate from this change). > > This is why I'd suggested just unconditionally clearing the bit. While > > When the hardware supports the STALL_SLOT event (again, I assume any > PMUv3 version hardware can support the event), and the guest's PMUVer > is older than v3p4, what is the reason why we want to clear the bit ? What's the value of the event w/o PMMIR_EL1? I agree there's no fundamental issue with letting it past, but I'd rather we start exposing the feature when we provide all the necessary detail. -- Thanks, Oliver
