[Qemu-devel] qemu-ga behavior on virtio-serial unplug

[Laszlo Ersek <lersek@redhat.com>]

Hello Michael,

this is with reference to
<https://bugzilla.redhat.com/show_bug.cgi?id=907733>.

Ever since the initial qemu-ga commit AFAICS an exception for
virtio-serial has existed, when reading EOF from the channel.

For isa-serial, EOF results in the client connection being closed. I
assume this exits the glib main loop somehow (otherwise qemu-ga would
just sit there and do nothing, as no further connections are accepted I
think).

For a unix domain socket, we can continue accepting new connections
after reading EOF.

For virtio-serial, EOF means "no host-side process is connected". In
this case we sleep a bit and go back to reading from the same fd (and
this turns into a sleep loop until the next host-side process connects).


Can we tell "virtio-serial port unplug" from "no host-side process"?
Because in the former case qemu-ga should really close the channel (and
maybe exit (*)), otherwise the unplug won't complete in the guest kernel.

According to Amit's comments in the RHBZ, at unplug a SIGIO is
delivered, and a POLLHUP event is reported. However,

(a) I think the glib iochannel abstraction doesn't allow us to tell
POLLHUP apart from reading EOF;

(b) delivery of an unhandled SIGIO seems to terminate the victim
process. qemu-ga doesn't seem to either catch or block SIGIO, which is
why I think a SIGIO signal is not sent in reality (maybe we should ask
for it first?)

... Actually I'm confused about this as well. The virtio-serial port
*is* opened with O_ASYNC (and on Solaris, it is replaced with an
I_SETSIG ioctl()). What's the reason for this? g_io_channel_unix_new()
doesn't seem to list it as a requirement, and qemu-ga doesn't seem to
handle SIGIO.

In any case we'd need a way to tell "host side close" from "port unplug".

(*) Then, there's the question what to do after unplug. Should we keep
reopening the same virtio-serial port (and sleeping a bit in-between)?
Or exit and let udev / systemd restart qemu-ga on any new virtio-serial
port, passing -p accordingly?

Thanks
Laszlo