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FEAT_PMUv3_TH (Armv8.8) is a new feature that allows conditional
counting of PMU events depending on how much the event increments on
a single cycle. Two new config fields for perf_event_open have been
added, and a PMU cap file for reading the max_threshold. See the second
commit message and the docs in the last commit for more details.
The feature is not currently supported on KVM guests, and PMMIR is set
to read as zero, so it's not advertised as available. But it can be
added at a later time. Writes to PMEVTYPER.TC and TH from guests are
already RES0.
The change has been validated on the Arm FVP model:
# Zero values, works as expected (as before).
$ perf stat -e dtlb_walk/threshold=0,threshold_compare=0/ -- true
5962 dtlb_walk/threshold=0,threshold_compare=0/
# Threshold >= 255 causes count to be 0 because dtlb_walk doesn't
# increase by more than 1 per cycle.
$ perf stat -e dtlb_walk/threshold=255,threshold_compare=2/ -- true
0 dtlb_walk/threshold=255,threshold_compare=2/
# Keeping comparison as >= but lowering the threshold to 1 makes the
# count return.
$ perf stat -e dtlb_walk/threshold=1,threshold_compare=2/ -- true
6329 dtlb_walk/threshold=1,threshold_compare=2/
