Re: [PATCH v6 08/12] virtio-serial-bus: add terminal resize messages

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 20, 2026 at 10:50:04AM +0100, Filip Hejsek wrote:
> On Mon, 2026-01-19 at 04:27 +0100, Filip Hejsek wrote:
> > Implement the part of the virtio spec that allows to notify the virtio
> > driver about terminal resizes. The virtio spec contains two methods to
> > achieve that:
> > 
> > For legacy drivers, we have only one port and we put the terminal size
> > in the config space and inject the config changed interrupt.
> > 
> > For multiport devices, we use the control virtqueue to send a packet
> > containing the terminal size. Note that old versions of the Linux kernel
> > used an incorrect order for the fields (rows then cols instead of cols
> > then rows), until it was fixed by commit 5326ab737a47278dbd16ed3ee7380b26c7056ddd.
> > 
> > As a result, when using a Linux kernel older than 6.15, the number of rows
> > and columns will be swapped.
> > 
> > Based on a patch originally written by Szymon Lukasz <noh4hss@gmail.com>,
> > but partially rewritten to fix various corner cases.
> > 
> > Signed-off-by: Szymon Lukasz <noh4hss@gmail.com>
> > Signed-off-by: Filip Hejsek <filip.hejsek@gmail.com>
> > ---
> >  hw/char/trace-events              |  1 +
> >  hw/char/virtio-serial-bus.c       | 76 +++++++++++++++++++++++++++++++++++++--
> >  hw/core/machine.c                 |  4 ++-
> >  include/hw/virtio/virtio-serial.h |  5 +++
> >  4 files changed, 83 insertions(+), 3 deletions(-)
> > 
> > [...]
> > 
> > diff --git a/include/hw/virtio/virtio-serial.h b/include/hw/virtio/virtio-serial.h
> > index 60641860bf..bda6d5312a 100644
> > --- a/include/hw/virtio/virtio-serial.h
> > +++ b/include/hw/virtio/virtio-serial.h
> > @@ -145,6 +145,9 @@ struct VirtIOSerialPort {
> >      bool host_connected;
> >      /* Do apps not want to receive data? */
> >      bool throttled;
> > +
> > +    /* Terminal size reported to the guest.  Only used for consoles. */
> > +    uint16_t cols, rows;
> >  };
> 
> I found a bug: after a migration, the guest is not informed about the
> new console size. I see two ways to fix this: either add the cols and
> rows fields to the migration stream, or always send the console size to
> the guest after migration, even if it might not have changed. Which do
> you prefer?

I prefer not adding a performance overhead to guests without
a good reason. We are already sending too much stuff to guests on
resume, let's not add more to this.

> Modifying the migration stream is somewhat annoying,
> because both versions will have to be supported, and also the device
> still uses legacy save/load functions rather than VMState.