Re: [PATCH v7 07/12] perf/x86: no counter allocation support

[Wei Wang <wei.w.wang@intel.com>]

On 07/08/2019 10:29 PM, Peter Zijlstra wrote:

Thanks for the comments.

>
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index 0ab99c7..19e6593 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -528,6 +528,7 @@ typedef void (*perf_overflow_handler_t)(struct perf_event *,
>>    */
>>   #define PERF_EV_CAP_SOFTWARE		BIT(0)
>>   #define PERF_EV_CAP_READ_ACTIVE_PKG	BIT(1)
>> +#define PERF_EV_CAP_NO_COUNTER		BIT(2)
>>   
>>   #define SWEVENT_HLIST_BITS		8
>>   #define SWEVENT_HLIST_SIZE		(1 << SWEVENT_HLIST_BITS)
>> @@ -895,6 +896,13 @@ extern int perf_event_refresh(struct perf_event *event, int refresh);
>>   extern void perf_event_update_userpage(struct perf_event *event);
>>   extern int perf_event_release_kernel(struct perf_event *event);
>>   extern struct perf_event *
>> +perf_event_create(struct perf_event_attr *attr,
>> +		  int cpu,
>> +		  struct task_struct *task,
>> +		  perf_overflow_handler_t overflow_handler,
>> +		  void *context,
>> +		  bool counter_assignment);
>> +extern struct perf_event *
>>   perf_event_create_kernel_counter(struct perf_event_attr *attr,
>>   				int cpu,
>>   				struct task_struct *task,
> Why the heck are you creating this wrapper nonsense?

(please see early discussions: https://lkml.org/lkml/2018/9/20/868)
I thought we agreed that the perf event created here don't need to consume
an extra counter.

In the previous version, we added a "no_counter" bit to perf_event_attr, and
that will be exposed to user ABI, which seems not good.
(https://lkml.org/lkml/2019/2/14/791)
So we wrap a new kernel API above to support this.

Do you have a different suggestion to do this?
(exclude host/guest just clears the enable bit when on VM-exit/entry,
still consumes the counter)

Best,
Wei