Re: [PATCH] virtio_console: Add support for multiple ports forgeneric guest and host communication

[Amit Shah]

On (Tue) Sep 22 2009 [21:15:49], Amit Shah wrote:
> On (Tue) Sep 22 2009 [12:14:04], Rusty Russell wrote:
> > On Sat, 12 Sep 2009 01:30:10 am Alan Cox wrote:
> > > > The interface presented to guest userspace is of a simple char
> > > > device, so it can be used like this:
> > > > 
> > > >     fd = open("/dev/vcon2", O_RDWR);
> > > >     ret = read(fd, buf, 100);
> > > >     ret = write(fd, string, strlen(string));
> > > > 
> > > > Each port is to be assigned a unique function, for example, the
> > > > first 4 ports may be reserved for libvirt usage, the next 4 for
> > > > generic streaming data and so on. This port-function mapping
> > > > isn't finalised yet.
> > > 
> > > Unless I am missing something this looks completely bonkers
> > > 
> > > Every time we have a table of numbers for functionality it ends in
> > > tears. We have to keep tables up to date and managed, we have to
> > > administer the magical number to name space.
> > 
> > The number comes from the ABI; we need some identifier for the different
> > ports.  Amit started using names, and I said "just use numbers"; they have
> > to be documented and agreed by all clients anyway.
> > 
> > ie. the host says "here's a port id 7", which might be the cut & paste
> > port or whatever.
> 
> Yeah; port 0 has to be reserved for a console (and then we might need
> to do a bit more for multiple consoles -- hvc operates on a 'vtermno',
> so we need to allocate them as well).

OK; how I solved this is: a new internal message between the guest and
the host. If a host is a console port, the host has to indicate that by
passing a message to the guest and the guest then hooks it up with hvc.

I'm also now maintaining a static vtermno int that's incremented
whenever consoles are added so that multiple ports are supported easily.

> Also, a 'name' property can be attached to ports, as has been suggested:
> 
> qemu ... -device virtconport,name=org.qemu.clipboard,port=3,...
> 
> spawns a port at id 3 and the guest will also place a file:
> 
> /sys/class/virtio-console/vcon3/name
> 
> which has "org.qemu.clipboard" as contents, so udev scripts could
> create a symlink:
> 
> /dev/vcon/org.qemu.clipboard -> /dev/vcon3

Using these concepts, this is the current patch that I have. Please give
it a good review and consider for inclusion.

		Amit