linux-kernel - Re: [PATCH] virtio: console: Make resizing compliant with virtio spec

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Message-ID: <f27debf87882df65574f21cfced31fecf1dd1da3.camel@kernel.org>
Date: Tue, 18 Mar 2025 15:25:54 +0100
From: Amit Shah <amit@...nel.org>
To: Maximilian Immanuel Brandtner <maxbr@...ux.ibm.com>, 
	linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev
Cc: arnd@...db.de, gregkh@...uxfoundation.org, brueckner@...ux.ibm.com, 
	schnelle@...ux.ibm.com, pasic@...ux.ibm.com
Subject: Re: [PATCH] virtio: console: Make resizing compliant with virtio
 spec

On Tue, 2025-03-18 at 11:07 +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
> > 
> > > 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).
> 
> Under section 5.3.6.2 multiport device operation for
> VIRTIO_CONSOLE_RESIZE the spec says the following
> 
> ```
> Sent by the device to indicate a console size change. value is
> unused.
> The buffer is followed by the number of columns and rows:
> 
> struct virtio_console_resize { 
>         le16 cols; 
>         le16 rows; 
> };
> ```

Indeed.


> It would be extremely surprising to me if the section `multiport
> device
> operation` does not document resize for multiport control messages,
> but
> rather config messages, especially as VIRTIO_CONSOLE_RESIZE is
> documented as a virtio_console_control event.

You're right.

I was mistaken in my earlier reply - I had missed this
virtio_console_resize definition in the spec.  So indeed there's a
discrepancy in Linux kernel and the spec's ordering for the control
message.

OK, that needs fixing someplace.  Perhaps in the kernel (like your
orig. patch), but with an accurate commit message.

Like I said, I don't think anyone is using this control message to
change console sizes.  I don't even think anyone's using multiple
console ports on the same device.

> In fact as far as I can tell this is the only part of the spec that
> documents resize. I would be legitimately interested in resizing
> without multiport and I would genuinely like to find out about how it
> could be used. In what section of the documentation could I find it?

See section 5.3.4 that describes `struct virtio_console_config` and
this note:

```
    If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver can
read the console dimensions from cols and rows. 
```

		Amit