Re: Replace GSource with AioContext for chardev

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/04/20 12:27, Daniel P. Berrangé wrote:
> Ignoring back compat, what would be our ideal CLI syntax ?
> 
> Current syntax is
> 
>   -chardev socket,id=charnet1,path=/tmp/vhost1.sock
>   -netdev vhost-user,chardev=charnet1,id=hostnet1 
> 
> Should we have an option that expresses a "SocketAddress" struct on the
> CLI ?
> 
>    -socket type=unix,path=/tmp/vhost1.sock,id=sock0
>    -netdev vhost-user,socket=sock0,id=hostnet1

I think this should be just a "-object socket" that under the covers
creates a QIOChannel.  There are also ideas of switching "-chardev" to
"-object"; we could do the reverse of Marc-André's suggestion, and have
"chardev=" take both a "chardev-foo" object or a QIOChannel object
(converting the latter to a socket-based chardev).

IOW, the new "-object socket" QOM type can act as both a chardev or a
QIOChannel factory.  The C side of that should not be hard.

Paolo

> IIUC, Marc-André is suggesting that we carry on using -chardev, but
> detect when it is a socket chardev, and then ignore chardev APIs and
> create a QIOChannel.  I can see some appeal in this as it provides a
> way to get all existing usage switched over, but I feel uneasy about
> sticking with -chardev forever, if we're not actually using a chardev.
> 
> We could do the magic -chardev -> -socket conversion though, for a
> short period of time to ease the transition.
> 
> We would have to
> 
>  1. Introduce the new -socket and add "socket=$id" to devices that need it
>  2. Deprecate -chardev with type != socket, with no repacement intended
>  3. Deprecate -chardev with type == socket, translating to -socket
>  ...wait 2 releases...
>  4. Delete support for "chardev=$id" from devices with "socket=$id"
> 
> The hardest part is probably deciding exactly which set of devices can
> be restricted to only sockets, and which must have the full range of
> chardev backends available.
> 
> Regards,
> Daniel
>