On Tue, 2026-01-20 at 11:22 +0100, Filip Hejsek wrote:
> On Tue, 2026-01-20 at 10:20 +0000, Daniel P. Berrangé wrote:
> > On Tue, Jan 20, 2026 at 11:16:51AM +0100, Filip Hejsek wrote:
> > > On Tue, 2026-01-20 at 10:10 +0000, Daniel P. Berrangé wrote:
> > > > On Tue, Jan 20, 2026 at 11:07:16AM +0100, Filip Hejsek wrote:
> > > > > On Tue, 2026-01-20 at 09:54 +0000, Daniel P. Berrangé wrote:
> > > > > > 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
> > > > > > > > <[email protected]>,
> > > > > > > > but partially rewritten to fix various corner cases.
> > > > > > > >
> > > > > > > > Signed-off-by: Szymon Lukasz <[email protected]>
> > > > > > > > Signed-off-by: Filip Hejsek <[email protected]>
> > > > > > > > ---
> > > > > > > > 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? 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.
> > > > > >
> > > > > > On the backend side, I'd consider migration to be equivalent to
> > > > > > closing
> > > > > > and re-opening the backend character device. That should imply
> > > > > > sending
> > > > > > a resize event on migration completion. I'm surprised the chardev
> > > > > > on
> > > > > > the dst isn't already triggering that when it gets connected, but
> > > > > > perhaps
> > > > > > that is too early & getting lost ?
> > > > >
> > > > > The virtio device caches the size and doesn't send a resize message if
> > > > > the size hasn't actually changed.
> > > >
> > > > If the size on the dest has not changed vs the size on the src, that's
> > > > fine surely ? We only need to tell the guest a new size if the dst
> > > > was different from the source after migration
> > >
> > > The current size is compared against previous size on the *dst*. We
> > > don't know the size on the src, because it is not sent in the migration
> > > stream.
> >
> > Oh, I see what you mean. In that case we should probably use a .post_load
> > hook to invalidate the dst cached size in some manner, so that the next
> > update from the backend is forced to be sent to the guest.
>
> I think just sending the cached size is simpler.
This fixes it:
@@ -806,6 +806,9 @@ static void virtio_serial_post_load_timer_cb(void *opaque)
port->host_connected);
}
vsc = VIRTIO_SERIAL_PORT_GET_CLASS(port);
+ if (vsc->is_console) {
+ send_console_resize(port);
+ }
if (vsc->set_guest_connected) {
vsc->set_guest_connected(port, port->guest_connected);
}
Best regards,
Filip
