On Tue, Jul 19, 2022 at 12:34:05PM +0100, Marc Zyngier wrote: > On Mon, 18 Jul 2022 16:49:10 +0100, > Ricardo Koller <ricarkol@xxxxxxxxxx> wrote: > > > > A chained event overflowing on the low counter can set the overflow flag > > in PMOVS. KVM does not set it, but real HW and the fast-model seem to. > > Moreover, the AArch64.IncrementEventCounter() pseudocode in the ARM ARM > > (DDI 0487H.a, J1.1.1 "aarch64/debug") also sets the PMOVS bit on > > overflow. > > Isn't this indicative of a bug in the KVM emulation? To be honest, the > pseudocode looks odd. It says: > > <quote> > if old_value<64:ovflw> != new_value<64:ovflw> then > PMOVSSET_EL0<idx> = '1'; > PMOVSCLR_EL0<idx> = '1'; > </quote> > > which I find remarkably ambiguous. Is this setting and clearing the > overflow bit? Or setting it in the single register that backs the two > accessors in whatever way it can? > > If it is the second interpretation that is correct, then KVM > definitely needs fixing I think it's the second, as those two "= '1'" apply to the non-chained counters case as well, which should definitely set the bit in PMOVSSET. > (though this looks pretty involved for > anything that isn't a SWINC event). Ah, I see, there's a pretty convenient kvm_pmu_software_increment() for SWINC, but a non-SWINC event is implemented as a single 64-bit perf event. Thanks, Ricardo > > M. > > -- > Without deviation from the norm, progress is not possible.
