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    SubjectRe: I.1 - System calls - ioctl

    > I/ General API comments
    >
    > 1/ System calls
    >
    > * ioctl()
    >
    > You have defined 5 ioctls() so far to operate on an existing
    > event. I was under the impression that ioctl() should not be
    > used except for drivers.
    >
    > How do you justify your usage of ioctl() in this context?

    We can certainly do a separate sys_perf_counter_ctrl() syscall - and
    we will do that if people think the extra syscall slot is worth it
    in this case.

    The (mild) counter-argument so far was that the current ioctls are
    very simple over "IO" attributes of counters:

    - enable
    - disable
    - reset
    - refresh
    - set-period

    So they could be considered 'IO controls' in the classic sense and
    act as a (mild) exception to the 'dont use ioctls' rule.

    They are not some weird tacked-on syscall functionality - they
    modify the IO properties of counters: on/off, value and rate. If
    they go beyond that we'll put it all into a separate syscall and
    deprecate the ioctl (which will have a relatively short half-time
    due to the tools being hosted in the kernel repo).

    This could happen right now in fact, if people think it's worth it.


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