Re: virtio-serial: A guest <-> host interface for simple communication

Rusty Russell
2009-06-25T11:33:04Z

On Wed, 24 Jun 2009 10:09:37 pm Amit Shah wrote:
> On (Wed) Jun 24 2009 [13:45:01], Rusty Russell wrote:
> > On Tue, 23 Jun 2009 10:12:31 pm Amit Shah wrote:
> > > Hello,
> > >
> > > Here are two patches. One implements a virtio-serial device in qemu
> > > and the other is the driver for a guest kernel.
> > >
> > > While working on a vmchannel interface that is needed for communication
> > > between guest userspace and host userspace, I saw that most of the
> > > interface can be abstracted out as a "serial" device with "ports".
> >
> > OK, I don't think the "naming" idea works though. A userspace user would
> > have to open each one in turn to get its name. I'd stick with numbers.
>
> What if an ioctl were added to get the port number from the port name?
> Userspace would do
> ioctl(fd, VIRTIO_SERIAL_GET_PORT_FROM_NAME, &nr);
> sprintf(port, "/dev/vmch%s", nr);
> fd2 = open(port, ...);

Yep, pretty ugly tho. If you use the "Amit Shah is in charge of port
numbering approach", then it's just:

fd = open("/dev/vmch<my-assigned-number>", ...);

Since you need to control names anyway, why not just use numbers?

> > You also don't have dynamic creation and removal, except by hotpluging
> > the entire device (which was on your requirements page).
>
> Actually we're more interested in hotplugging ports than the device
> itself ("Dynamic channel creation").

Exactly, which you don't seem to have.

> > what ports exist. Register on the change interrupt to get updates. Drop
> > the control vq entirely.
>
> If the ioctl mentioned above were added, it would justify the control
> vq, right?

Yes. Or put another way, simplifying the interface to use assigned port
numbers would simplify the implementation, use, and specification of the device
:)

Cheers,
Rusty.