Re: [PATCH v3 1/3] perf: Align user space counter reading with code

[Ingo Molnar <mingo@kernel.org>]

* Vince Weaver <vincent.weaver@maine.edu> wrote:

> On Tue, 19 Jul 2022, Ian Rogers wrote:
> 
> > Align the user space counter reading documentation with the code in
> > perf_mmap__read_self. Previously the documentation was based on the perf
> > rdpmc test, but now general purpose code is provided by libperf.
> > 
> > Signed-off-by: Ian Rogers <irogers@google.com>
> > ---
> >  include/uapi/linux/perf_event.h       | 35 +++++++++++++++++----------
> >  tools/include/uapi/linux/perf_event.h | 35 +++++++++++++++++----------
> >  2 files changed, 44 insertions(+), 26 deletions(-)
> > 
> > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> > index d37629dbad72..6826dabb7e03 100644
> > --- a/include/uapi/linux/perf_event.h
> > +++ b/include/uapi/linux/perf_event.h
> > @@ -538,9 +538,13 @@ struct perf_event_mmap_page {
> >  	 *
> >  	 *     if (pc->cap_usr_time && enabled != running) {
> >  	 *       cyc = rdtsc();
> > -	 *       time_offset = pc->time_offset;
> >  	 *       time_mult   = pc->time_mult;
> >  	 *       time_shift  = pc->time_shift;
> > +	 *       time_offset = pc->time_offset;
> > +	 *       if (pc->cap_user_time_short) {
> > +	 *         time_cycles = pc->time_cycles;
> > +	 *         time_mask = pc->time_mask;
> > +	 *       }
> 
> From what I've been told, and from what perf_mmap__read_self() actually 
> does, many of these MMAP fields need to be accessed by READ_ONCE()
> (a GPLv2 only interface) to be correct.

BTW., for this purpose I guess we could add a READ_ONCE() variant to perf 
tooling that reimplements it with more relaxed licensing, so that the 
headers & sample code can be included in GPL-incompatible projects?

READ_ONCE() isn't a super complicated primitive, and we've always been 
permissive with simple primitives.

Thanks,

	Ingo