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

[Peter Zijlstra]

On Tue, Jun 18, 2024 at 08:10:40AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> 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.