On 4/24/2024 1:02 AM, Mingwei Zhang wrote:
Maybe, (just maybe), it is possible to do PMU context switch at vcpu
boundary normally, but doing it at VM Enter/Exit boundary when host is
profiling KVM kernel module. So, dynamically adjusting PMU context
switch location could be an option.
If there are two VMs with pmu enabled both, however host PMU is not
enabled. PMU context switch should be done in vcpu thread sched-out path.
If host pmu is used also, we can choose whether PMU switch should be
done in vm exit path or vcpu thread sched-out path.
host PMU is always enabled, ie., Linux currently does not support KVM
PMU running standalone. I guess what you mean is there are no active
perf_events on the host side. Allowing a PMU context switch drifting
from vm-enter/exit boundary to vcpu loop boundary by checking host
side events might be a good option. We can keep the discussion, but I
won't propose that in v2.
I suspect if it's really doable to do this deferring. This still makes
host lose the most of capability to profile KVM. Per my understanding,
most of KVM overhead happens in the vcpu loop, exactly speaking in
VM-exit handling. We have no idea when host want to create perf event to
profile KVM, it could be at any time.
I guess we are off topic. Sean's suggestion is that we should put
"perf" and "kvm" together while doing the context switch. I think this
is quite reasonable regardless of the PMU context switch location.
To execute this, I am thinking about adding a parameter or return
value to perf_guest_enter() so that once it returns back to KVM, KVM
gets to know which counters are active/inactive/cleared from the host
side. Knowing that, KVM can do the context switch more efficiently.
Thanks.
-Mingwei
