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