[Qemu-devel] qemu guest agent spins in poll/nanosleep(100ms) when nothing is listening on host

[Daniel P. Berrange]

I've been doing some experimentation with the QEMU guest agent and have
noticed that when nothing is connected on the host side of the virtio
serial channel, the guest agent just spins in a pool/sleep(100ms) loop.
I know you'd ordinarily expect some mgmt app in the host to be listening
to the other end of the channel, but it still seems suboptimal to have
to spin in a loop like this when nothing is listening, constantly causing
wakeups in an otherwise idle guest.

Looking at the qemu-ga.c code I see two places where it might handle
a poll event and then sleep, when nothing is on the other end of the
virtio serial socket.


   case G_IO_STATUS_AGAIN:
        /* virtio causes us to spin here when no process is attached to
         * host-side chardev. sleep a bit to mitigate this
         */
        if (s->virtio) {
            usleep(100*1000);
        }
        return true;

   ....


    } else if (strcmp(s->method, "virtio-serial") == 0) {
        /* we spin on EOF for virtio-serial, so back off a bit. also,
         * dont close the connection in this case, it'll resume normal
         * operation when another process connects to host chardev
         */
        usleep(100*1000);
        goto out_noclose;
    }


I get the feeling that this kind of problem inherant in the use of any
virtio-serial channel, in the same way you can't detect EOF for a regular
serial device channel either. Given that virtio-serial is a nice paravirt
device, is there anything we can do to it, to allow better handling of
EOF by applications ?

Or perhaps there is some way to make use of epoll() in edge-triggered
mode to detect it already, because IIUC, edge-triggered mode should only
fire once for the EOF condition, and then not fire again until something
in the host actually sends some data ?

Of course glib's event loop doesn't support edge-triggered events/epoll,
but perhaps we could just call epoll() directly in the event handler,
instead of the usleep() call ?

Regards,
Daniel
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