kqueue(2) kevents for jails

[Fabian Freyer]

On 1/4/19 9:29 PM, Konstantin Belousov wrote:
> On Fri, Jan 04, 2019 at 09:11:58PM +0100, Fabian Freyer wrote:
>> On 1/4/19 5:14 PM, Konstantin Belousov wrote:
>>> No, kevent(2) is not suitable mechanism to notify about jail state changes.
>>> If anything in the existing system can be reused for such notifications,
>>> it is devctl(4) notifications which are handled by devd(8).  Look at the
>>> man pages and for existing notifications in kernel code, e.g.
>>> sys/kern/kern_conf.c notify*() for how devfs does it.
>>
>> Can any running binary subscribe to devd(8) events or does that require
>> a configuration change in /etc/devd.conf?
> 
> Only one reader is supported, effectively.  devctl(4) tries to limit opens
> naively.  But then even if you have the file descriptor and fork or pass
> it over unix domain socket, single event can be only read by one reader.
> 

Ah, I see, thanks! Is there any other nice notification mechanism that a 
process could 'subscribe' to to be notified of an event?

I am still a bit confused as to why knotify would be such a bad fit, 
maybe you could expand a bit on that?

> Not least because jail creation/destruction is relatively low frequency
> events with potentially rich secondary information that should be attached
> to them.  Kevents are high-frequency, high-performance kind of events,

Does this mean they cannot nicely be used for lower-frequency things? 
I'm thinking of situations where jails may get spawned e.g. 
per-network-request.

> and only naturally tied to file descriptors. 

according to kevent(2),

EVFILT_PROC         Takes the process ID to monitor as the identifier

so there's also cases where it isn't tied to a file descriptor, but some 
other descriptor (pid's don't seem to be too different to jid's?)

> There were lot of bugs in
> integration of kevents with e.g. processes notifications, and API is
> still somewhat racy

Is this a kevents issue or an integration problem?

In the end, might it be a good idea to add devctl(4) notifications as 
well as kevent(2)?