[SeaBIOS] [SeaVGABIOS] No VBE modes for card on PCIe Root Port

[Kevin Locke]

Hi All,

With QEMU 7.0.0 and SeaBIOS 1.16.0, virtual machines with a PCIe
graphics card connected to a PCIe Root Port, rather than the Root
Complex, as recommended by the QEMU PCI Express Guidelines[1], are
unable to use VESA BIOS Extensions display modes.  For example,

qemu-system-x86_64 \
    -no-user-config \
    -nodefaults \
    -machine q35,accel=kvm \
    -m 1G \
    -cdrom "$iso" \
    -device VGA

works well and can use many VBE display modes, while

qemu-system-x86_64 \
    -no-user-config \
    -nodefaults \
    -machine q35,accel=kvm \
    -m 1G \
    -cdrom "$iso" \
    -device pcie-root-port,id=pci.1,bus=pcie.0 \
    -device VGA,bus=pci.1

only shows a few text display modes.  This is the case for VGA,
qxl-vga, and virtio-vga devices.  `info qtree` shows the device as
`type: PCIE` in both configurations.  High-resolution graphics modes
are available when using UEFI (OVMF) in both configurations.

Am I correct in assuming this is a limitation of SeaVGABIOS?  Is this
expected behavior, or a bug?  If it's unlikely to change soon, would
it make sense for me to propose documenting this limitation in the
QEMU PCI Express Guidelines?

I'd appreciate any insights you might have.

Thanks,
Kevin

[1]: https://github.com/qemu/qemu/blob/v7.0.0/docs/pcie.txt