On Fri, 14 Feb 2020 18:36:13 +0000 Marc Zyngier <maz@xxxxxxxxxx> wrote: > It is at times necessary to prevent a guest from being able to sample > certain events if multiple CPUs share resources such as a cache level. > In this case, it would be interesting if the VMM could simply prevent > certain events from being counted instead of simply not exposing a PMU. > > Given that most events are not architected, there is no easy way > to designate which events shouldn't be counted other than specifying > the raw event number. > > Since I have no idea whether it is better to use an event whitelist > or blacklist, the proposed API takes a cue from the x86 version and > allows either allowing or denying counting of ranges of events. > The event space being pretty large (16bits on ARMv8.1), the default > policy is set by the first filter that gets installed (default deny if > we first allow, default allow if we first deny). > > The filter state is global to the guest, despite the PMU being per > CPU. I'm not sure whether it would be worth it making it CPU-private. > > Anyway, I'd be interesting in comments on how people would use this. > I'll try to push a patch against kvmtool that implement this shortly > (what I have currently is a harcoded set of hacks). I now have a small extension to kvmtool allowing a --pmu-filter option to be passed on the command line (see [1]). I've also pushed out an update[2] to the kernel side of things, making the filtering of the cycle counter consistent and documenting that neither SW_INCR nor CHAIN could be filtered with this mechanism (but this is of course up for discussion). Thanks, M. [1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/kvmtool.git/commit/?h=pmu-filter [2] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pmu-event-filter -- Jazz is not dead. It just smells funny...
