Re: [RFC PATCH 5/6] bpf: Implement function bpf_read_pmu() that get the selected hardware PMU conuter

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 17, 2015 at 08:27:43PM +0800, Wangnan (F) wrote:
> I think we can check the limitation in BPF program.

You typically do not want to rely on your program for correctness.

> What about this:
> 
>  event must on current CPU or must be on current process. If not,
>  bpf_read_pmu() should simply return an error.

OK, that's workable. That enforces the constraints outside of the
program itself.

> With current design it is easy to implement, and users can still control
> it through bpf map.
> 
> But what if we really want cross-cpu PMU accessing? Impossible?

Under the assumption that the eBPF program is called from tracing, and
therefore from any context (task, softirq, irq and nmi), yes impossible.

You cannot do (synchronous) IPIs from either IRQ context or with IRQs
disabled. And both are valid trace contexts.