Trace event filtering

Hi, all

I have a simple question about filters in libtraceevent. It looks like
libtraceevent parses and matches filters in user-space, but kernel
already implements filtering in kernel-space which is better because
it should prevent events from generating, if not matched. So why
doesn't libtraceevent use kernel filters?

I am trying to filter out non-error block_rq_complete events, clearly
I don't want to see rasdaemon woken up every time for error==0 case,
which is literally all the time.

BTW, the user-space filters have slightly different syntax with
kernel-space filters, at least the regex matching looks slightly
different to me.

Am I miss anything here?

Thanks!