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Subjectperf_event_open() ABI compatability

So I think this might be revisiting an issue that has come up before, but
we're having backward compatability issues with PAPI and libpfm4 and
the perf_event_open() system call.

If a user specifies exclude_guest=1 on an older kernel that doesn't
support it, we get the awesome EINVAL error return code and it often
takes hours to track down the cause.

Now in theory the ABI is maintained via the "size" field. So you can
figure out the size of the attr struct by setting an invalid size
and then getting E2BIG with size set to the value the kernel expects.

This doesn't help with exclude_guest though, as that's in the giant union
in the middle of the attr, and there's absolutely no mechanism at all
to tell when that has been extended.

Is there any solution to all of this, except having to carry around a big
table of kernel version numbers for when features were added?

Ideally we would somehow want E2BIG returned plus the size of __reserved_1
if the value of __reserved_1 is not zero. I suppose at this point in the
game it's too late for this to be much help and we're going to have to
work around the problem forever anyway.


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