Re: [RFC PATCH v2 2/3] perf/x86/intel/pt: Introduce and export pt_get_curr_event()

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2022-09-22 8:58 a.m., Wang, Wei W wrote:
> On Thursday, September 22, 2022 8:34 PM, Liang, Kan wrote:
>>> To solve the problem, introduce and export pt_get_curr_event() for KVM
>>> to get current pt event.
>>
>> I don't think the current pt event is created by KVM. IIUC, the patch basically
>> expose the event created by the other user to KVM. That doesn't sounds
>> correct.
> 
> Yes, that's the host PT event running on the current CPU. Not created by KVM.
> 
>>
>> I think it should be perf's responsibility to decide which events should be
>> disabled, and which MSRs should be switched. Because only perf can see all the
>> events. The users should only see the events they created.
> 
> For other pmu cases, yes. For PT, its management is simpler than other pmu
> resources and PT PMU is much simpler. It doesn't have a scheduler to manage
> events.
>

Right, but I think we'd better to create a simpler scheduler (just for
two events) in the PT driver, since you have two co-existing PT events
now, one is from the host and the other is from the guest. I don't think
it's KVM's responsibility to schedule events.

So I think the process should be something as below.

- Let KVM create a PT event if the guest request one.
- In VM-entry, just invoke the perf_event_enable_local(guest).
  The PT driver should schedule out the host event or whatever events
and schedule in the guest event.
- In VM-exit, just invoke the perf_event_disable_local(guest).
  The PT driver should schedule in the host event or whatever events and
schedule out the guest event.

I still don't think we want to expose the host event.

Thanks,
Kan

> I think the usage here is similar to the CPU thread scheduling case. When we have
> CPUs isolated from the CPU scheduler, i.e. no scheduler for those CPUs, basically
> we rely on users to ping tasks to those CPUs (e.g. taskset).
> 
> For the commit log, probably we don't need those KVM details here. Simplify a bit:
> 
> Add a function to expose the current running PT event to users. One usage is in KVM,
> it needs to get and disable the running host PT event before VMEnter to the guest and
> resumes the event after VMexit to host.