Re: [PATCH v4 00/10] virtio-console: notify about the terminal size

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

On Fri, Sep 12, 2025 at 04:41:05AM -0400, Michael S. Tsirkin wrote:
> On Fri, Sep 12, 2025 at 05:39:45AM +0200, Filip Hejsek wrote:
> > The goal of this series is to have a resizable terminal into a guest
> > without having to set up networking and using, e.g. ssh.
> > 
> > The virtio spec allows a virtio-console device to notify the guest about
> > terminal resizes in the host. Linux Kernel implements the driver part of
> > the spec. This series implement the device part in QEMU.
> > 
> > This series adds support for a resizable terminal if a virtio console
> > device is connected to the stdio backend.
> > 
> > This series also introduces resize messages that can be sent over QMP to
> > notify QEMU about the size of the terminal connented to some chardev.
> > In the libvirt setting, it will allow to implement a resizable terminal
> > for virsh console and other libvirt clients.
> > 
> > This patch series was originally authored by Szymon Lukasz and submitted
> > to qemu-devel about 5 years ago. The previous submission can be found at
> > <https://lists.nongnu.org/archive/html/qemu-devel/2020-06/msg09591.html>.
> > I have updated the patches to be compatible with latest master and made
> > a few small changes of my own, including the addition of Windows support.
> > 
> > Probably the most important change I made is the swapping of rows and
> > cols fields in resize messages. I would like to hear some feedback on
> > this change from reviewers. The problem is that for a long time, the
> > Linux kernel used a different field order from what was specified in the
> > virtio spec. The kernel implementation was apparently merged around 2010,
> > while the virtio spec came in 2014, so when a previous version of this
> > patch series was being discussed here on this mailing list in 2020, it
> > was decided that QEMU should match the Linux implementation, and ideally,
> > the virtio spec should be changed.
> > 
> > However, recently, the Linux kernel implementation was changed to conform
> > to the spec: <https://git.kernel.org/linus/5326ab737a47278dbd16ed3ee7380b26c7056ddd>.
> > As a result, to be compatible with latest kernel releases, QEMU needs to
> > also use the field order matching the specification. I have changed the
> > patch to use the spec-compliant order, so it works correctly with latest
> > kernels now.
> >
> 
> Well this is not in any release yet. If you want me to revert that
> one, let me know ASAP. Maximilian?
> 
> > That leaves the issue of older kernels. There are about 15 years' worth
> > of kernel versions with the swapped field order, including the kernel
> > currently shipped in Debian stable. The effects of the swapped dimensions
> > can sometimes be quite annoying - e.g. if you have a terminal with
> > 24 rows, this will be interpreted as 24 columns, and your shell may limit 
> > line editing to this small space, most of which will be taken by your
> > prompt. The patch series in its current form provides no way to disable
> > the console size functionality,
> 
> Well I see the console-size property, no?
> 
> > so users may end up worse off than if
> > the functionality were not implemented at all.
> 
> If we want to keep the Linux patch, the straight forward way is to send
> the fix to stable@ then poke at Debian at al to fix their kernels.
> 
> Another option is to make the property default to off, have
> management turn it on when guest is up to date.
> 
> But it really sounds like we should revert that Linux patch.
> I posted a revert, pls comment.

Oh no, I was confused. This was in 6.15 and 6.16 :(


Pls ignore this, I will reply separately.




> > PS: One of the patches adds one additional noop switch case
> >     to a bunch of files. I didn't include the maintainers of these files
> >     in the Cc list. I hope that's OK. :)
> > 
> > v4:
> > changed order of rows and cols fields
> > added support for terminal size on Windows
> > trace event is also emitted for legacy (non-multiport) drivers
> > minor fixes required because of changes in QEMU (DECLARE_INSTANCE_CHECKER, qmp-example)
> > updated version numbers ('Since: 10.2', hw_compat_10_1)
> > 
> > v3:
> > add resize messages over QMP, as suggested by Daniel
> > 
> > v2:
> > fix adding a new virtio feature bit to the virtio console device
> > 
> > ---
> > Filip Hejsek (1):
> >       char-win-stdio: add support for terminal size
> > 
> > Szymon Lukasz (9):
> >       chardev: add cols, rows fields
> >       chardev: add CHR_EVENT_RESIZE
> >       chardev: add qemu_chr_resize()
> >       char-mux: add support for the terminal size
> >       main-loop: change the handling of SIGWINCH
> >       char-stdio: add support for the terminal size
> >       qmp: add chardev-resize command
> >       virtio-serial-bus: add terminal resize messages
> >       virtio-console: notify the guest about terminal resizes
> > 
> >  backends/cryptodev-vhost-user.c   |  1 +
> >  chardev/char-fe.c                 | 13 ++++++++
> >  chardev/char-mux.c                | 18 ++++++++++-
> >  chardev/char-stdio.c              | 30 +++++++++++++++++++
> >  chardev/char-win-stdio.c          | 19 ++++++++++++
> >  chardev/char.c                    | 26 ++++++++++++++++
> >  hw/block/vhost-user-blk.c         |  1 +
> >  hw/char/terminal3270.c            |  1 +
> >  hw/char/trace-events              |  1 +
> >  hw/char/virtio-console.c          | 63 ++++++++++++++++++++++++++++++++++++---
> >  hw/char/virtio-serial-bus.c       | 43 ++++++++++++++++++++++++--
> >  hw/core/machine.c                 |  4 ++-
> >  hw/ipmi/ipmi_bmc_extern.c         |  1 +
> >  hw/scsi/vhost-user-scsi.c         |  1 +
> >  hw/usb/ccid-card-passthru.c       |  1 +
> >  hw/usb/dev-serial.c               |  1 +
> >  hw/usb/redirect.c                 |  1 +
> >  hw/virtio/vhost-user-base.c       |  1 +
> >  hw/virtio/vhost-user-scmi.c       |  1 +
> >  include/chardev/char-fe.h         | 10 +++++++
> >  include/chardev/char.h            |  7 +++++
> >  include/hw/virtio/virtio-serial.h |  5 ++++
> >  include/qemu/main-loop.h          |  4 +++
> >  monitor/hmp.c                     |  1 +
> >  monitor/qmp.c                     |  1 +
> >  net/passt.c                       |  1 +
> >  net/vhost-user.c                  |  1 +
> >  qapi/char.json                    | 22 ++++++++++++++
> >  ui/curses.c                       | 11 +++----
> >  util/main-loop.c                  | 21 +++++++++++++
> >  30 files changed, 298 insertions(+), 13 deletions(-)
> > ---
> > base-commit: 190d5d7fd725ff754f94e8e0cbfb69f279c82b5d
> > change-id: 20250912-console-resize-96c42140ba08
> > 
> > Best regards,
> > -- 
> > Filip Hejsek <filip.hejsek@gmail.com>