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 2022-09-26 1:24 p.m., Jim Mattson wrote:
> On Mon, Sep 26, 2022 at 9:55 AM Liang, Kan <kan.liang@linux.intel.com> wrote:
>>
>>
>>
>> On 2022-09-22 10:42 a.m., Wang, Wei W wrote:
>>> On Thursday, September 22, 2022 10:10 PM, Peter Zijlstra wrote:
>>>> On Thu, Sep 22, 2022 at 01:59:53PM +0000, Wang, Wei W wrote:
>>>>> On Thursday, September 22, 2022 9:35 PM, Peter Zijlstra
>>>>>> On Thu, Sep 22, 2022 at 12:58:49PM +0000, Wang, Wei W wrote:
>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>> You cannot just kill a host event like that. If there is a host
>>>>>> event, the guest looses out.
>>>>>
>>>>> OK. The intention was to pause the event (that only profiles host
>>>>> info) when switching to guest, and resume when switching back to host.
>>>>
>>>> If the even doesn't profile guest context, then yes. If it does profile guest
>>>> context, you can't.
>>>
>>> Seems better to add this one:
>>
>> If the guest host mode is enabled, I think the PT driver should not
>> allow the perf tool to create a host event with !exclude_guest.
> 
> While I agree that guest events should generally have priority over
> host events, this is not consistent with the way "normal" PMU events
> are handled.
Only when the two events try to access a resource at the same time, we
have to decide whether priority an event or share between events. But I
don't think this is the case here.

From my understanding of the host-guest mode, the host PT event never
traces the guest no matter whether the guest enables PT. So when
VM-entry, there is only a guest PT event or no event. If so, I think the
perf tool should warn the user if they try to create a host event with
!exclude_guest, since the host event never traces a guest.


Thanks,
Kan