Hi,
Hmm, why? The user asked for a serial console. Whenever that is provided by seabios builtin support or the sgabios rom doesn't really matter, no?
Surely it has ABI implications for the guest if we stop putting the sgabios option ROM into memory - changing layout of ROMS
Because of that changing -device sga behavior on the qemu side must be tied to the qemu machine type.
Also, the question is what seabios should do in case both are active. Which I think can happen today with libvirt if you configure a virtual machine without vga (libvirt passes -machine graphics=off which activates builtin serial console) and with "<bios useserial='yes'/>" (enables sgabios rom).
Libvirt doesn't use '-machine graphics=off' AFAIK, only '- nographic'.
I think that is pretty much the same. Setting "graphichs=off" is one of the effects of passing -nographic, and the other effects (like setting up default serial + monitor in a different way) don't happen due to libvirt also using -nodefaults.
cheers, Gerd