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

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

On Fri, Sep 12, 2025 at 09:50:30AM +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/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?
> 
> 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.
> 
> > > 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.
> 
> What about other non-Linux guest OS that may have correctly
> implemented the virtio spec ?
> 
> At least FreeBSD appears to /not/ implemenmt resize at all:
> 
>   https://github.com/freebsd/freebsd-src/blob/main/sys/dev/virtio/console/virtio_console.c#L884
> 
> Do we have a Windows impl of virtio-console with resize support ?

Windows seems to ignore it:

        case VIRTIO_CONSOLE_RESIZE:
            TraceEvents(TRACE_LEVEL_INFORMATION, DBG_PNP, "VIRTIO_CONSOLE_RESIZE id = %d\n", cpkt->id);
            break;



> Any other places we should check ?
> 
> With regards,
> Daniel
> -- 
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