Re: [RFC] bpf: lbr: enable reading LBR from tracing bpf programs

[Peter Zijlstra]

On Wed, Aug 18, 2021 at 04:46:32PM +0000, Song Liu wrote:

> > Urgghhh.. I so really hate BPF specials like this.
> 
> I don't really like this design either. But it does show that LBR can be
> very useful in non-PMI scenario. 
> 
> > Also, the PMI race
> > you describe is because you're doing abysmal layer violations. If you'd
> > have used perf_pmu_disable() that wouldn't have been a problem.
> 
> Do you mean instead of disable/enable lbr, we disable/enable the whole 
> pmu? 

Yep, that way you're serialized against PMIs. It's what all of the perf
core does.

> > I'd much rather see a generic 'fake/inject' PMI facility, something that
> > works across the board and isn't tied to x86/intel.
> 
> How would that work? Do we have a function to trigger PMI from software, 
> and then gather the LBR data after the PMI? This does sound like a much
> cleaner solution. Where can I find code examples that fake/inject PMI?

We don't yet have anything like it; but it would look a little like:

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...

> There is another limitation right now: we need to enable LBR with a 
> hardware perf event (cycles, etc.). However, unless we use the event for 
> something else, it wastes a hardware counter. So I was thinking to allow
> software event, i.e. dummy event, to enable LBR. Does this idea sound 
> sane to you?

We have a VLBR dummy event, but I'm not sure it does exactly as you
want. However, we should also consider Power, which also has the branch
stack feature.

You can't really make a software event with LBR on, because then it
wouldn't be a software event anymore. You'll need some hybrid like
thing, which will be yuck and I suspect it needs arch support one way or
the other :/