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

[Laszlo Ersek <lersek@redhat.com>]

On 06/20/13 15:31, Amit Shah wrote:
> On (Wed) 19 Jun 2013 [13:17:57], Laszlo Ersek wrote:

>> In any case we'd need a way to tell "host side close" from "port unplug".
> 
> Will POLLHUP|POLLERR help, along with error returns on read() and
> write()?

I think so:
- read() == 0  --> host side disconnected,
- read() == -1, equivalently POLLERR --> unplug (or other error)
- write() == -1 / errno == EPIPE: host side disconnected,
- write() == -1 / errno == EIO (or ENXIO or EINVAL): unplug

I think the current code could be adapted to such a scheme gracefully.
(Regarding the error codes on write(), I just made them up, but you get
the idea.)

On hot-unplug we could bail out to a more external loop that tries to
reopen the same device, or just exit and leave the restart to udev/systemd.


If possible I would like to avoid SIGIO. SIGIO is basically a non-queued
(= can be pending or not pending; any realtime queueing variant is
limited in depth hence useless) and edge triggered readiness
notification. It isn't portable and requires extra hoops to jump through
just to get the file descriptor and to tell a read event from a write event.

Although it's possible to base level triggered readiness on top of it
(by setting read & write readiness booleans on SIGIO and clearing them
on the respective -1/EAGAIN, while blocking SIGIO carefully), I think
that's quite distant from our current event loop. Hence we should remove
O_ASYNC (and the Solaris equivalent too) with the same fell swoop.

Thanks
Laszlo