On Fri, Apr 05, 2019 at 01:43:08PM +0100, Will Deacon wrote: > On Thu, Apr 04, 2019 at 08:33:51PM +0100, Andrew Murray wrote: > > On Thu, Apr 04, 2019 at 05:21:28PM +0100, Will Deacon wrote: > > > On Thu, Mar 28, 2019 at 10:37:31AM +0000, Andrew Murray wrote: > > > > +exclude_kernel > > > > +-------------- > > > > + > > > > +This attribute excludes the kernel. > > > > + > > > > +The kernel runs at EL2 with VHE and EL1 without. Guest kernels always run > > > > +at EL1. > > > > + > > > > +This attribute will exclude EL1 and additionally EL2 on a VHE system. > > > > > > I find this last sentence a bit confusing, because it can be read to imply > > > that if you don't set exclude_kernel and you're in a guest on a VHE system, > > > then you can profile EL2. > > > > Yes this could be misleading. > > > > However from the perspective of the guest, when exclude_kernel is not set we > > do indeed allow the guest to program it's PMU with ARMV8_PMU_INCLUDE_EL2 - and > > thus the statement above is correct in terms of what the kernel believes it is > > doing. > > > > I think these statements are less confusing if we treat the exception levels > > as those 'detected' by the running context (e.g. consider the impact of nested > > virt here) - and we if ignore what the hypervisor (KVM) does outside (e.g. > > stops counting upon switching between guest/host, translating PMU filters in > > kvm_pmu_set_counter_event_type etc, etc). This then makes this document useful > > for those wishing to change this logic (which is the intent) rather than those > > trying to understand how we filter for EL levels as seen bare-metal. > > > > With regards to the example you gave (exclude_kernel, EL2) - yes we want the > > kernel to believe it can count EL2 - because one day we may want to update > > KVM to allow the guest to count it's hypervisor overhead (e.g. host kernel > > time associated with the guest). > > If we were to support this in the future, then exclude_hv will suddenly > start meaning something in a guest, so this could be considered to be an ABI > break. > > > I could write some preface that describes this outlook. Alternatively I could > > just spell out what happens on a guest, e.g. > > > > "For the host this attribute will exclude EL1 and additionally EL2 on a VHE > > system. > > > > For the guest this attribute will exclude EL1." > > > > Though I'm less comfortable with this, as the last statement "For the guest this > > attribute will exclude EL1." describes the product of both > > kvm_pmu_set_counter_event_type and armv8pmu_set_event_filter which is confusing > > to work out and also makes an assumption that we don't have nested virt (true > > for now at least) and also reasons about bare-metal EL levels which probably > > aren't that useful for someone changing this logic or understanding what the > > flags do for there performance analysis. > > > > Do you have a preference for how this is improved? > > I think you should be explicit about what is counted. If we don't count EL2 > when profiling in a guest (regardless of the exclude_*) flags, then we > should say that. By not documenting this we don't actually buy ourselves > room to change things in future, we should have an emergent behaviour which > isn't covered by our docs. OK no problem, I'll update this. Andrew Murray > > Will _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
