Re: Compatibility between -device sga and -machine graphics=off

[Gerd Hoffmann <kraxel@redhat.com>]

On Wed, Sep 08, 2021 at 05:08:08PM +0100, Daniel P. Berrangé wrote:
> Given the libvirt XML snippet
> 
>   <os>
>     ...
>     <bios useserial="yes"/>
>     ...
>   </os>
> 
> libvirt QEMU driver will always format
> 
>   -device sga
> 
> Libguestfs uses this syntax, so we need to make sure it still works
> in future even if 'sga' is disabled or removed in a QEMU build in
> favour of SeaBIOS' built-in support.

Just replacing '-device sga' with '-machine graphics=off' in case sga is
not available should work fine.

serial console support in seabios is available for quite a while
(merged in 2017, seabios 1.11 in rhel-7 has it), so switching over
unconditionally is possibly an option too.  Not sure what the libvirt
backward compatibility policy is though.

>  1. Graphical display only, no serial port, BIOS to graphical display
>  2. Serial port only, no graphics, BIOS to nowhere
>  3. Serial port only, no graphics, BIOS to serial
>  4. Graphical display, serial port, BIOS only to graphical display
>  5. Graphical display, serial port, BIOS to graphical display + serial

Should all work fine.

> If I do 'info mtree' though, I do see a different memory layout
> when changing from SGA to graphics=off

> -    00000000000cb000-00000000000cdfff (prio 1000, ram): alias kvmvapic-rom @pc.ram 00000000000cb000-00000000000cdfff
> +    00000000000ca000-00000000000ccfff (prio 1000, ram): alias kvmvapic-rom @pc.ram 00000000000ca000-00000000000ccfff

probably sgabios.bin is loaded to ca000 when enabled.

> On non-x86 emulators I see graphics=off has semantic effects beyond
> just controlling whether the firmware prints to the serial or not
> though.

It's been a while, but yes, IIRC on ppc this is passed to the linux
kernel somehow (device tree?) so it also affects the default console
device used by linux.

But sgabios is x86-only anyway so that should not be a problem here.

HTH & take care,
  Gerd