[libvirt-users] How can we achieve vga emulation over a serial port in libvirt

[Martin Kletzander]

On Wed, Feb 28, 2018 at 03:10:08PM +0800, Meina Li wrote:
>Hi
>
>For the latest seabios version, it said: Support for vga emulation over a
>serial port in SeaBIOS (sercon).
>
>So I want to know how can I find the application of this feature in
>libvirt? And whether my understanding is correct?
>There are no related instruction on the website: https://libvirt.org/
>
>My understanding:
>(1) The feature means: we can set this the IO address of a serial port to
>"video" element to enable SeaBIOS' VGA adapter emulation on the given
>serial port.

Looks like it, however the question is if that's somehow set as a default or
not.  According to the docs for SeaBIOS it says:

sercon-port: Set this to the IO address of a serial port to enable SeaBIOS' VGA
             adapter emulation on the given serial port.

However looking at current QEMU master, there is no such setting being done (I
just did simple git grep sercon-port).  However it looks like other options are
being set using fw_cfg_add_file, so it should be possible to set that using
something like:

-fw_cfg name=etc/sercon-port,string=0x3F8

but when I tried (with seabios 1.11.0), I realized I'm not sure how to see
whether it works or not.  How would I try that, easily?

Anyway, if the above is the case, we don't _yet_ have an option in libvirt's
XML.  We will need to add support for fw_cfg, I just didn't, for now.

>(2) Libvirt XML examples and with this xml the guest can start successfully:
>    <controller type='virtio-serial' index='0'>
>      <address type='pci' domain='0x0000' bus='0x00' slot='0x06'
>function='0x0'/>
>    </controller>
>    <video>
>    <model type='vga' vram='16384' heads='1' primary='yes'/>
>    <address type='virtio-serial' controller='0' bus='0' port='3'/>
>    </video>
>

I'm not sure how the inner workings of this are supposed to work, so I can't
comment on the XML structure.

>Thank in advance.
>
>
>
>
>Best Regards
>Meina Li

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