On Mon, 26 Mar 2012 10:40:54 +0100, Will Deacon <will.deacon at arm.com> wrote: > [removed some dead email addresses from CC] Thanks! > On Fri, Mar 23, 2012 at 11:17:07PM +0000, Rusty Russell wrote: > > I had assumed that as we trend towards a common cross-platform kernel, > > we should be relying on feature registers where available, not the > > model. > > The PMU is very tightly coupled with the CPU and it is necessary to know > exactly which CPU you have so that you can map microarchitectural events to > their identifiers. This is not discoverable in the architecture [PMUv2 > provides some support for discovering the status of architected events, but > that is still not enough and is only present on A7 and A15 so far] so that's > why we probe the CPU instead. OK. > Now, if everything was device-tree based then we could simply use a > different binding for each CPU but since we support perf on non-DT > platforms, probing the CPU type is the best solution. I would like to avoid > the probing code if we are initialised from DT, but I've not got round to it > yet (this would be useful for big.LITTLE). I'll be interested to see how you encode the mapping of microarchitectural events to their identifiers here. > > > Would you be able to advertise 0 event counters instead and treat the PMU > > > largely as RAZ/WI? The only tricky bit I can see is the cycle counter, which > > > is mandated by the presence of a PMU, however this could also just RAZ > > > initially. > > > > Yes, that's the effect of the current emulation hack. Far better not to > > lie to the guest, however, than get weird results from profiling. > > I'd rather lie to the guest than modify it just to avoid emulating the > device correctly. Even then, the profiling results wouldn't be especially > weird -- everything would be 0, which is a lot better than random > numbers. Sure, but nowhere as neat an experience as having "no hardware support available" printed, and nicely defined failure mode :( When I get back in May, I'll look at how hard it will to be implement this properly, if noone else does. Thanks, Rusty. -- How could I marry someone with more hair than me? http://baldalex.org
