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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