Re: [tip:perf/core] perf: Fix broken union in ' struct perf_event_mmap_page'

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 17, 2013 at 04:23:25PM -0400, Vince Weaver wrote:
> >  include/uapi/linux/perf_event.h | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> > index 00d8274..0041aed 100644
> > --- a/include/uapi/linux/perf_event.h
> > +++ b/include/uapi/linux/perf_event.h
> > @@ -375,9 +375,11 @@ struct perf_event_mmap_page {
> >  	__u64	time_running;		/* time event on cpu */
> >  	union {
> >  		__u64	capabilities;
> > -		__u64	cap_usr_time  : 1,
> > -			cap_usr_rdpmc : 1,
> > -			cap_____res   : 62;
> > +		struct {
> > +			__u64	cap_usr_time		: 1,
> > +				cap_usr_rdpmc		: 1,
> > +				cap_____res		: 62;
> > +		};
> >  	};

> This patch somehow breaks the perf-ABI.

Difficult call that..

> If I take a program that reads "mmap->cap_usr_rdpmc" and compile it
> against the new header with this change (say from 3.12-rc1)
> and then run it on an old kernel (say 3.11) then I get "0" for
> cap_usr_rdpmc.
> 
> If I take the same program and recompile against the old (without this 
> patch) header and run it on 3.11, I get the expected "1" value.
> 
> So something about this changed the bit pattern in an incompatible 
> fashion.

Which was the entire point of the change. Previously cap_usr_time and
cap_usr_rdpmc were the same bit which clearly cannot be right.

With the change they're consecutive bits in the capabilities word.

Should we preserve completely broken things that don't work?