On 07/09/2019 07:45 PM, Peter Zijlstra wrote:
+ * config: Actually this field won't be used by the perf core
+ * as this event doesn't have a perf counter.
+ * sample_period: Same as above.
If it's unused; why do we need to set it at all?
OK, we'll remove the unused fields.
+ * sample_type: tells the perf core that it is an lbr event.
+ * branch_sample_type: tells the perf core that the lbr event works in
+ * the user callstack mode so that the lbr stack will be
+ * saved/restored on vCPU switching.
Again; doesn't make sense. What does the user part have to do with
save/restore? What happens when this vcpu thread drops to userspace for
an assist?
This is a fake event which doesn't run the lbr on the host.
Returning to userspace doesn't need save/restore lbr, because userspace
wouldn't change those lbr msrs.
The event is created to save/restore lbr msrs on vcpu switching.
Host perf only do this save/restore for "user callstack mode" lbr event,
so we
construct the event to be "user callstack mode" to reuse the host lbr
save/restore.
An alternative method is to have KVM vcpu sched_in/out callback
save/restore those msrs, which don't need to create this fake event.
Please let me know if you prefer the second method.
Best,
Wei