Re: [Qemu-devel] questions about host side of virtio-serial

[Richard W.M. Jones]

On Wed, Jul 30, 2014 at 12:52:41PM -0600, Chris Friesen wrote:
> Hi,
> 
> I'm working on a native user of virtio-serial (ie, not going via the
> qemu guest agent).
> 
> The information at "http://www.linux-kvm.org/page/Virtio-serial_API";
> does a good job of describing the guest side of things, but has very
> little information about the host side of things.

You might also want to read the libguestfs source code, since
libguestfs is a major and long-time user of virtio-serial.

In particular these files:

  src/launch-direct.c
  src/proto.c
  src/conn-socket.c
  daemon/proto.c
  src/launch-libvirt.c  # if interested in using virtio-serial from libvirt

> In particular, assuming that the host side is using a chardev mapped
> to a unix socket:
> 
> 1) Is there any way for the host app to get information about
> whether or not the guest is reading the messages?  (i.e. logically
> equivalent to getting POLLHUP in the guest when the host app
> disconnects.)

No, I don't believe that is possible.  It acts like a real serial port
and throws away bytes when no one is listening (on both ends).

> 2) Suppose the host sends a large message.  The guest app reads a
> portion of the message, then crashes.  We respawn the guest app and
> start reading again, but now we're in the middle of a message of
> arbitrary size.  Is there a recommended technique to re-sync the
> host and guest?

AFAIK that's either very difficult or impossible.  Maybe with some
kind of self-synchronizing protocol, or if you ran SLIP/PPP on top of
the raw virtio-serial channel?

In the libguestfs case we wouldn't even try to go there -- if
something in the VM crashes we completely recreate the virtual machine
from scratch.

> 3) Same as 2, but the guest sending to the host and the host app
> crashing partway through.
> 
> 4) If nothing in the guest is reading the data, how much data can
> the host send before it will get an error?

It won't get an error - the sender will block.  Except on ARM where
there is a race condition in virtio-mmio causing writes to be thrown
away (https://bugs.launchpad.net/qemu/+bug/1224444).

> Is there a way to adjust this?

Not as far as I know.

Rich.

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