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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Sep 15, 2025 at 10:41:44AM +0200, Maximilian Immanuel Brandtner wrote:
> On Fri, 2025-09-12 at 09:50 +0100, Daniel P. Berrangé wrote:
> > On Fri, Sep 12, 2025 at 04:41:02AM -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/5326ab737a47278dbd16ed3ee7380b26c70
> > > > 56ddd>.
> > > > 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?
> > 
> > At least in the case of libvirt managed VMs, this series does
> > nothin by default, as they won't be using the 'stdio' chardev,
> > they'll require libvirt to first wire up the new QMP command,
> > and then apps using libvirt to call it. So in that sense, it'll
> > take a while before the effects are seen by users.
> 
> Correct me if I'm wrong on this, but shouldn't the 'pty' chardev also
> be able to take advantage of the same size change mechanisms as the
> 'stdio' chardev (receiving SIGWINCH and being able to use the ioctl
> TIOCGWINSZ)? If so the work for the 'stdio' chardev should probably be
> replicated for the 'pty' chardev.

Yes, if using a suitable client app, it ought to work for
'pty' I think.


With regards,
Daniel
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