Re: [Qemu-devel] [PATCH v2 0/3] seabios: add serial console support

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/07/2016 12:00, Gerd Hoffmann wrote:
> One option would be to continue using sgabios.bin in fw_cfg.  But
> instead of loading and executing it activate the build-in serial console
> if we find the file.  That would make the switch completely transparent
> to upper layers.  But it is quite hackish of course ...

OpenBIOS uses FW_CFG_NOGRAPHIC for this, which is the negation of
qdev_get_machine()->enable_graphics and set by both "-nographic" and
"-machine graphics=no" ("-nographic" is a shortcut for "-display none
-machine graphics=no").  The default is "-machine graphics=yes".

As long as QEMU doesn't enable both FW_CFG_NOGRAPHIC and sgabios.bin, it
should be fine.  For example, if SeaBIOS's serial console is made
accessible with "-machine graphics=no", then:

* QEMU's "-device sga" should do nothing if machine->enable_graphics is set.

* For old machine types, QEMU should hardcode FW_CFG_NOGRAPHIC to
false(*) so that sgabios is never blocked.

For new machine types, SGABIOS will still be accessible with "-device
sga".  New machine types depend on SeaBIOS that has a serial console,
otherwise "-nographic -device sga" breaks.

Paolo

   (*) Hardcoding FW_CFG_NOGRAPHIC to false doesn't match what old QEMU
       does, but provides the best backwards-compatibility:

       - old->new: Because old QEMU provides sgabios, you have to
                   include sgabios.bin in the destination too or
                   migration fails.  After migration, the BIOS comes
                   from old QEMU and itnever looks at FW_CFG_NOGRAPHIC,
                   so the value doesn't matter---we just make it false.

       - new->old: Because old QEMU expects sgabios to be there, you
                   have to include sgabios.bin.  The BIOS comes from
                   new QEMU and looks at FW_CFG_NOGRAPHIC.  To avoid
                   double hooking FW_CFG_NOGRAPHIC has to be false.