[libvirt] Seabios can't boot from a disk behind i82801b11-bridge device (was Re: [PATCH 6/7] qemu: add dmi-to-pci-bridge controller)

On 08/02/2013 12:55 PM, Laine Stump wrote: Sigh. (yeah, I know I've been saying that a lot lately :-) I've just now figured out that seabios doesn't recognize any disk device that is connected anywhere behind the i82801b11-bridge device. So if you have a virtio-blk-pci device that is attached in this manner (the way these patches are setup to work): pcie.0->i82801b11-bridge->pci-bridge->virtio-blk-pci or even like this (precluding later hot-plug, but at least still not violating the hardware specs): pcie.0->i82801b11-bridge->virtio-blk-pci seabios won't see the disk, and it won't boot. However, if you ignore the fact that the pci-bridge device requires a standard PCI slot (*not* PCIe) and shouldn't be directly plugged into pcie.0 and do this (which is allowed by qemu even though it strictly speaking *shouldn't* be): pcie.0->pci-bridge->virtio-blk-pci then seabios recognizes the disk and is able to boot from it. So I see three options for taking care of of this problem 1) leave this libvirt code as it is, and petition seabios to add support for recognizing i82801b11-bridge devices and booting from disks that are connected behind them drawback: we will remain unable to create working Q35 machines with libvirt until seabios is fixed. 2) modify these patches to add the pci-bridge device directly to pcie.0 and not add any dmi-to-pci-bridge in the middle (this will end up encoded in the domain's config) drawback: when seabios is fixed and libvirt switches to the "proper" model of the hardware, there will be many configurations already out in the field using the old model which will either need to be modified, or libvirt will need to continue allowing/supporting that setup. 3) modify these patches to substitute a pci-bridge for the i82801b11-bridge when someone requests a dmi-to-pci-bridge. advantage: when seabios is fixed, configs already in the field will continue to work drawback: live migration of a domain from old to new will fail in some strange as-yet-to-be-determined fashion. Also, until seabios is fixed and we remove the patch to replace all i82801b11-bridge devices with pci-bridge at runtime, there will be no testing of the i82801b11-bridge code in qemu (at least not via libvirt-managed guests). Presumably if I did option (2) or (3), this would be changed back at a later date when seabios is fixed to recognize i82801b11-bridge properly. Of course we will want libvirt to work properly no matter what is the version of seabios/qemu, so we'll have to figure out a way to determine if the seabios bugfix is in or not. Does anyone have an opinion on which of these is preferred (or a different idea)? I'm tending towards option (3), since that will cause the least headaches later (there won't be a bunch of stale configs that need to be updated). Whatever I do, it will be done in a separate patch from this, because this patch does implement things in the "correct" manner, and should stand; any hack to workaround the problem in seabios should be a separate object in the git history.