RE: Proposed systemtap access to perfmon hardware

["Stone, Joshua I" <joshua.i.stone@intel.com>]

William Cohen wrote:
> It is and open question what the counters default are; do they start
> running by default or have to be explicitly started.

This could be solved with an extension on the probe declaration.  For
example, the sensible default is probably to start the counters, but we
could allow a ".paused" to override that default.

> If they are started by default, where exactly are they running?
> Beginning of begin probe? End of begin probe?

I think the perfmon setup needs to happen before all begin probes, so
that the handle can be accessed within begin probes.  I don't know that
it really matters when you actually start the counters, but I would lean
towards putting that after.

A side note about handles - it might be useful to add a language
semantic to make it easier to capture handles.  There are actions that
would make sense for handles of other probe types besides perfmon,
especially being able to dynamically enable/disable kprobes and timers.


Josh