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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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