[Qemu-devel] Re: virtio-serial semantics for binary data and guest agents

[Amit Shah]

On (Wed) 23 Feb 2011 [08:31:52], Michael Roth wrote:
> On 02/22/2011 10:59 PM, Amit Shah wrote:
> >On (Tue) 22 Feb 2011 [16:40:55], Michael Roth wrote:
> >>If something in the guest is attempting to read/write from the
> >>virtio-serial device, and nothing is connected to virtio-serial's
> >>host character device (say, a socket)
> >>
> >>1. writes will block until something connect()s, at which point the
> >>write will succeed
> >>
> >>2. reads will always return 0 until something connect()s, at which
> >>point the reads will block until there's data
> >>
> >>This makes it difficult (impossible?) to implement the notion of
> >>connect/disconnect or open/close over virtio-serial without layering
> >>another protocol on top using hackish things like length-encoded
> >>payloads or sentinel values to determine the end of one
> >>RPC/request/response/session and the start of the next.
> >>
> >>For instance, if the host side disconnects, then reconnects before
> >>we read(), we may never get the read()=0, and our FD remains valid.
> >>Whereas with a tcp/unix socket our FD is no longer valid, and the
> >>read()=0 is an event we can check for at any point after the other
> >>end does a close/disconnect.
> >
> >There's SIGIO support, so host connect-disconnect notifications can be
> >caught via the signal.
> 
> I recall looking into this at some point....but don't we get a SIGIO
> for read/write-ability in general?

I don't get you -- the virtio_console driver emits the SIGIO signal
only when the host side connects or disconnects.  See

http://www.linux-kvm.org/page/Virtio-serial_API

So whenever you receive a SIGIO, poll() in the signal handler for all
fds of interest and whichever has POLLIN set is writable.  Whichever
has POLLHUP set is not.  If you maintain previous state of the fd
(before signal), you can figure out if something happened on the host
side.

> So you still need some way
> differentiate, say, readability from a disconnect/EOF, and the
> read()=0 that could determine this is still racing with host-side
> reconnects.

> >Also, nonblocking reads/writes will return -EPIPE if the host-side
> >connection is not up.
> 
> But we still essentially need to poll() for a host-side disconnected
> state, which is still racy since they may reconnect before we've
> done a read/write that would've generated the -EPIPE. It seems like
> what we really need is for the FD to be invalid from that point
> forward.

This would go against (or abuse) a chardev interface.  It would
effectively treat a host-side port close as a hot-unplug event.

> Also, I focused more on the guest-side connect/disconnect detection,
> but as Anthony mentioned I think the host side shares similar
> limitations as well. AFAIK once we connect to the chardev that FD
> remains valid until the connected process closes it, and so races
> with the guest side on detecting connect/disconnect events in a
> similar manner. For the host side it looks like virtio-console has
> guest_close/guest_open callbacks already that we could potentially
> use...seems like it's just a matter of tying them to the chardev...
> basically having virtio-serial's guest_close() result in a close()
> on the corresponding chardev connection's FD.

Yes, this could be used.

However, the problem with that will be that the chardev can't be
opened again (AFAIR) and a new chardev will have to be used.


So if this is done on both the sides, the race will be eliminated but
the expectation that a chardev port is just a serial port will be
broken and we'll try to bake in some connection layer on top of it.
That wasn't the original idea.  We could extend this, but a better way
to achieve this could be a library on either side to abstract these
details off.

                Amit