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? -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html