On Thu, Aug 19, 2021 at 04:46:20PM +0000, Song Liu wrote:

quoted

void perf_inject_event(struct perf_event *event, struct pt_regs *regs)
{
	struct perf_sample_data data;
	struct pmu *pmu = event->pmu;
	unsigned long flags;

	local_irq_save(flags);
	perf_pmu_disable(pmu);

	perf_sample_data_init(&data, 0, 0);
	/*
	 * XXX or a variant with more _ that starts at the overflow
	 * handler...
	 */
	__perf_event_overflow(event, 0, &data, regs);

	perf_pmu_enable(pmu);
	local_irq_restore(flags);
}

But please consider carefully, I haven't...

Hmm... This is a little weird to me. 
IIUC, we need to call perf_inject_event() after the software event, say
a kretprobe, triggers. So it gonna look like:

  1. kretprobe trigger;
  2. handler calls perf_inject_event();
  3. PMI kicks in, and saves LBR;

This doesn't actually happen. I overlooked the fact that we need the PMI
to fill out @data for us.

  4. after the PMI, consumer of LBR uses the saved data;

Normal overflow handler will have data->br_stack set, but I now realize
that the 'psuedo' code above will not get that. We need to somehow get
the arch bits involved; again :/

However, given perf_inject_event() disables PMU, we can just save the LBR
right there? And it should be a lot easier? Something like:

  1. kretprobe triggers;
  2. handler calls perf_snapshot_lbr();
     2.1 perf_pmu_disable(pmu);
     2.2 saves LBR 
     2.3 perf_pmu_enable(pmu);
  3. consumer of LBR uses the saved data;

What is the downside of this approach? 

It would be perf_snapshot_branch_stack() and would require a new
(optional) pmu::method to set up the branch stack.

And if we're going to be adding new pmu::methods then I figure one that
does the whole sample state might be more useful.