On Fri, Aug 11, 2023 at 07:05:20PM +0100, Marc Zyngier wrote: > Huang Shijie reports that, when profiling a guest from the host > with a number of events that exceeds the number of available > counters, the reported counts are wildly inaccurate. Without > the counter oversubscription, the reported counts are correct. > > Their investigation indicates that upon counter rotation (which > takes place on the back of a timer interrupt), we fail to > re-apply the guest EL0 enabling, leading to the counting of host > events instead of guest events. Seems to me, it's not clear for why the counter rotation will cause the issue. In the example shared by Shijie in [1], the cycle counter is enabled for both host and guest, and cycle counter is a dedicated event which does not share counter with other events. Even there have counter rotation, it should not impact the cycle counter. I mean if we cannot explain clearly for this part, we don't find the root cause, and this patch (and Shijie's patch) just walks around the issue. Thanks, Leo [1] https://lore.kernel.org/lkml/20230810072906.4007-1-shijie@xxxxxxxxxxxxxxxxxxxxxx/
