Message-ID: <12d78b156c65130c60503b3925b4440d570fa245.camel@linux.ibm.com>
Date: Wed, 05 Mar 2025 13:15:02 +0100
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Maximilian Immanuel Brandtner <maxbr@...ux.ibm.com>,
        Amit Shah	
 <amit@...nel.org>, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux.dev
Cc: arnd@...db.de, gregkh@...uxfoundation.org, brueckner@...ux.ibm.com,
        pasic@...ux.ibm.com
Subject: Re: [PATCH] virtio: console: Make resizing compliant with virtio
 spec

On Wed, 2025-03-05 at 13:13 +0100, Niklas Schnelle wrote:
> On Wed, 2025-03-05 at 10:53 +0100, Maximilian Immanuel Brandtner wrote:
> > On Mon, 2025-03-03 at 12:54 +0100, Amit Shah wrote:
> > > On Tue, 2025-02-25 at 10:21 +0100, Maximilian Immanuel Brandtner
> > > wrote:
> > > > According to the virtio spec[0] the virtio console resize struct
> > > > defines
> > > > cols before rows. In the kernel implementation it is the other way
> > > > around
> > > > resulting in the two properties being switched.
> > > 
> > > Not true, see below.
> > > 
> > > > While QEMU doesn't currently support resizing consoles, TinyEMU
> > > 
> > > QEMU does support console resizing - just that it uses the classical
> > > way of doing it: via the config space, and not via a control message
> > > (yet).
> > > 
> > > https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/char/virtio_console.c#n1787
> > > 
> > > https://lists.nongnu.org/archive/html/qemu-devel/2010-05/msg00031.html
> > 
> > I didn't know about this patch-set, however as of right now QEMU does
> > not set VIRTIO_CONSOLE_F_SIZE, never uses VIRTIO_CONSOLE_RESIZE, and
> > resizing is never mentioned in hw/char/virtio-console.c or
> > hw/char/virtio-serial-bus.c. Suffice to say I don't see any indicating
> > of resize currently being used under QEMU. Perhaps QEMU supported
> > resizing at one point, but not anymore. If you disagree please send me
> > where the resizing logic can currently be found in the QEMU source
> > code. I at least was unable to find it.
> > 
> > > 
> > > > diff --git a/drivers/char/virtio_console.c
> > > > b/drivers/char/virtio_console.c
> > > > index 24442485e73e..9668e89873cf 100644
> > > > --- a/drivers/char/virtio_console.c
> > > > +++ b/drivers/char/virtio_console.c
> > > > @@ -1579,8 +1579,8 @@ static void handle_control_message(struct
> > > > virtio_device *vdev,
> > > >   break;
> > > >   case VIRTIO_CONSOLE_RESIZE: {
> > > >   struct {
> > > > - __u16 rows;
> > > >   __u16 cols;
> > > > + __u16 rows;
> > > >   } size;
> > > >  
> > > >   if (!is_console_port(port))
> > > 
> > > This VIRTIO_CONSOLE_RESIZE message is a control message, as opposed
> > > to
> > > the config space row/col values that is documented in the spec.
> > > 
> > > Maybe more context will be helpful:
> > > 
> > > Initially, virtio_console was just a way to create one hvc console
> > > port
> > > over the virtio transport.  The size of that console port could be
> > > changed by changing the size parameters in the virtio device's
> > > configuration space.  Those are the values documented in the spec. 
> > > These are read via virtio_cread(), and do not have a struct
> > > representation.
> > > 
> > > When the MULTIPORT feature was added to the virtio_console.c driver,
> > > more than one console port could be associated with the single
> > > device.
> > > Eg. we could have hvc0, hvc1, hvc2 all as part of the same device. 
> > > With this, the single config space value for row/col could not be
> > > used
> > > for the "extra" hvc1/hvc2 devices -- so a new VIRTIO_CONSOLE_RESIZE
> > > control message was added that conveys each console's dimensions.
> > > 
> > > Your patch is trying to change the control message, and not the
> > > config
> > > space.
> > > 
> > > Now - the lack of the 'struct size' definition for the control
> > > message
> > > in the spec is unfortunate, but that can be easily added -- and I
> > > prefer we add it based on this Linux implementation (ie. first rows,
> > > then cols).
> > > 
> > > But note that all this only affects devices that implement multiport
> > > support, and have multiple console ports on a single device.  I don't
> > > recall there are any implementations using such a configuration.
> > > 
> > > ... which all leads me to ask if you've actually seen a
> > > misconfiguration happen when trying to resize consoles which led to
> > > this patch.
> > > 
> > >  Amit
> > 
> > I'm working on implementing console resizing for virtio in QEMU and
> > Libvirt. As SIGWINCH is raised on the virsh frontend the new console
> > size needs to be transfered to QEMU (in my RFC patch via QOM, which
> > then causes QEMU to trigger a virtio control msg in the chr_resize
> > function of the virtio-console chardev). (The patch-set should make its
> > way unto the QEMU mailing-list soon). The way I implemented it QEMU
> > sends a resize control message where the control message has the
> > following format:
> > 
> > ```
> > struct {
> >     le32 id;    // port->id
> >     le16 event; // VIRTIO_CONSOLE_RESIZE
> >     le16 value; // 0
> >     le16 cols;  // ws.ws_col
> >     le16 rows;  // ws.ws_row
> > }
> > ```
> > 
> > This strongly seems to me to be in accordance with the spec[0]. It
> > resulted in the rows and cols being switched after a resize event. I
> > was able to track the issue down to this part of the kernel. Applying
> > the patch I sent upstream, fixed the issue.
> > As of right now I only implemented resize for multiport (because in the
> > virtio spec I was only able to find references to resizing as a control
> > message which requires multiport. In your email you claimed that config
> > space resizing exists as well. I was only able to find references to
> > resizing as a control message in the spec. I would love to see what
> > part of the spec you are refering to specifically, as it would allow me
> > to implement resizing without multiport as well). 
> > It seems to me that either the spec or the kernel implementation has to
> > change. If you prefer changing the spec that would be fine by me as
> > well, however there seems to be no implementation that uses the linux
> > ordering and Alpine seems to patch their kernel to use the spec
> > ordering instead (as described in the initial email)(this was really
> > Niklas Schnelle's finding so for further questions I would refer to
> > him).
> 
> I don't think this was patched in the (official) alpine kernel. What
> happened is that I tested TinyEMU[0] with the kernel + initrd from the
> JSLinux site and that has working console resizing. In the TinyEMU code
> this is implemented in TinyEMU/virtio.c:virtio_console_resize_event():
> 
> void virtio_console_resize_event(VIRTIODevice *s, int width, int height)
> {
>     /* indicate the console size */
>     put_le16(s->config_space + 0, width);
>     put_le16(s->config_space + 2, height);
> 
>     virtio_config_change_notify(s);
> }
> 
> On second look this indeed seems to use the config space. It writes
> first the width then height which matches config_work_handler(). But
> like Maximilian I could only find the VIRTIO_CONSOLE_RESIZE message
> mechanism in the spec, also with width (cols) then height (rows) but
> not matching the kernel struct changed by this patch.
> 

Forgot to note, the idea that Alpine was patched came because the
kernel used in TinyEMU has '-dirty' in the local version so we were
wondering if it was patched for this. But seeing the
config_work_handler() it's probably just using that.

Thanks,
Niklas

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