On Wed, 2022-06-08 at 12:49 +0200, Gerd Hoffmann wrote: > On Tue, Jun 07, 2022 at 09:37:21AM -0600, Kevin Locke wrote: >> 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, > >> Am I correct in assuming this is a limitation of SeaVGABIOS? > > Well, sort of. There are two ways to reach the registers needed for > the extended display modes, via ioports 0x01ce,0x01cf or via mmio bar > (see docs/specs/standard-vga.txt in qemu source tree). > > The ioport access is not forwarded over pci(e) bridges, the special > exception bit for vga register access applies to the classic vga > registers only not the vbe registers. > > vgabios functions must be able to run in real mode, so it uses the > ioports, because they are easily reachable. > > ovmf driver runs in 32bit or 64bit anyway, so using the mmio bar is no > problem and the driver prefers that if available. Therefore it is > possible to plug vga devices into a pcie root port (although I think > it doesn't work for qxl due to mmio bar not being available there). > > > If it's unlikely to change soon, would > > it make sense for me to propose documenting this limitation in the > > QEMU PCI Express Guidelines? > > Yes, it makes sense to add an exception note there. VGA devices should > be placed on the root bus to make sure they are fully functional > (libvirt does that by default btw). > > When using UEFI it is possible to pick a device without vga > compatibility (i.e. bochs-display or virtio-gpu-pci). The legacy vga > stuff (text mode and lowres graphics) wouldn't be used anyway ...
Thanks Gerd, I really appreciate the explanation! I've tried to incorporate your explanation in a patch to the QEMU PCI Express Guidelines that I posted to qemu-devel for discussion.[1] Feel free to follow up there with any improvements or if I've misunderstood anything. Re: libvirt. Interestingly, libvirt only appears to do so for the first/primary display device.[2] I ran into this issue after adding a second video device to a VM in virt-manager, then removing the first, which left the only video device on a Root Port leading to the missing display modes. I'm not sure if it's a bug worth filing/fixing. Let me know if you have any thoughts on it. Thanks again, Kevin [1]: https://lore.kernel.org/qemu-devel/922cc3081ff9c986188f881ef4d1cf15bd3adf48.1654739990.git.ke...@kevinlocke.name/T/#u [2]: https://gitlab.com/libvirt/libvirt/-/blob/v8.4.0/src/qemu/qemu_domain_address.c#L1825-1858 _______________________________________________ SeaBIOS mailing list -- seabios@seabios.org To unsubscribe send an email to seabios-le...@seabios.org
