Re: [Qemu-devel] qemu-ga virtio-serial socket clarification

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Oct 25, 2016 at 7:14 PM, Matt Broadstone <mbroadst@gmail.com> wrote:
> I've been attempting an experimental qemu agent using a node.js daemon on
> the host side, and have run into an issue I was hoping someone here might
> be able to help with:
>
> Using libvirt I've set up a 'unix' channel for a domain using virtio-serial
> (the same way you would for the existing qemu agent) with the name
> 'test.agent', in order to bypass libvirt taking ownership of the domain
> socket. This works as expected, and so does the following test:
>
>  - [host] $ echo "testing" | nc -U
> /var/lib/libvirt/qemu/channel/target/domain-T40001/test.agent
>  - [guest] $ cat -v < /dev/virtio-ports/test.agent
>
> Then I tried the same test, converting the host->guest communication to
> node.js:
>
> 'use strict';
> const net = require('net');
> const socketPath =
> '/var/lib/libvirt/qemu/channel/target/domain-T40001/test.agent';
> let socket = net.createConnection(socketPath);
> socket.write('testing');
>
> In this case the data makes it across to the guest, however until I
> explicitly close the socket on the sender side (`socket.write('testing', ()
> => socket.end())`) both sides block indefinitely. I understand closing the
> socket brings the node example to parity with the netcat one, however after
> perusing the qemu-ga and libvirt repositories it looks like glib's io
> channels are being used on a single socket, and effectively handling
> bidirectional data.
>
> Is this the expected behavior?
>
> This would seem to imply that normal async communication over the domain
> socket is somehow different in the virtio-serial case (as in I can't
> maintain a duplex socket, but would rather have to juggle opening and
> closing read/write sockets). In my research I came across another similar
> project: https://github.com/xolox/python-negotiator, which requires two
> channels: one for host->guest communication, and another for guest->host
> communication, likely because of this very issue.

virtio-serial is full-duplex.

Please post the receive side test program you are using.

Stefan