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 :/
