Re: [RESEND PATCH 08/12] perf/x86: Extend event update interface

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

On 2024-06-20 4:38 a.m., Peter Zijlstra wrote:
> On Tue, Jun 18, 2024 at 08:10:40AM -0700, kan.liang@linux.intel.com wrote:
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> The current event update interface directly reads the values from the
>> counter, but the values may not be the accurate ones users require. For
>> example, the sample read feature wants the counter value of the member
>> events when the leader event is overflow. But with the current
>> implementation, the read (event update) actually happens in the NMI
>> handler. There may be a small gap between the overflow and the NMI
>> handler. The new Intel PEBS counters snapshotting feature can provide
>> the accurate counter value in the overflow. The event update interface
>> has to be updated to apply the given accurate values.
>>
>> Pass the accurate values via the event update interface. If the value is
>> not available, still directly read the counter.
>>
>> For some cases, e.g., intel_update_topdown_event, there could be more
>> than one counter/register are read. Using u64 * rather than u64 as the
>> new parameter.
> 
> The stronger argument is that there is no special value to distinguish
> between there begin an argument and there not being one. You cannot have
> !val, because 0 might be a valid rdpmc() value.

Indeed. I will update the description.

Thanks,
Kan