Messages in this thread | | | Date | Tue, 17 Sep 2013 16:35:30 -0400 (EDT) | From | Vince Weaver <> | Subject | Re: [tip:perf/core] perf: Fix broken union in ' struct perf_event_mmap_page' |
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On Tue, 17 Sep 2013, Vince Weaver wrote:
> > This patch somehow breaks the perf-ABI. > > 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.
To follow up, the original case:
union { __u64 capabilities; __u64 cap_usr_time : 1, cap_usr_rdpmc : 1, cap_____res : 62; };
Then mmap->usr_rdpmc=1; gets assembled as
400420: 80 0d 11 05 20 00 02 orb $0x2,0x200511(%rip)
The new version
union { __u64 capabilities; struct { __u64 cap_usr_time : 1, cap_usr_rdpmc : 1, cap_usr_time_zero : 1, cap_____res : 61; }; };
mmap->usr_rdpmc=1; gets assembled as
400427: 80 0d 02 05 20 00 01 orb $0x1,0x200502(%rip)
note the difference in the or value.
Vince
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