Re: [PATCH v2] docs: add PCIe root bus for VGA compat guideline

[Gerd Hoffmann]

  Hi,

> I think documenting the issue with VBE registers would be helpful.
> Doing so with a recommendation for how to avoid the issue seems even
> better.  Would a recommendation to attach a device which supports VBE
> to the Root Complex if VBE will be used by the guest make sense?

Yes.  This affects all vga-compatible devices emulated by qemu except cirrus.

> As you noted, applying the recommendation to all VGA compatible
> devices may be too broad.  I'm not sure whether it makes sense to
> recommend attaching VGA compatible devices to the Root Complex to
> avoid the complexity of the VGA exception bits, or if that is a
> non-issue.  In fact, if I understand correctly, it may make sense to
> recommend attaching VGA compatible devices to separate PCI bridges if
> the VM will have multiple VGA compatible devices so that the guest can
> perform VGA arbitration.

Yes.  Also guest drivers for pci-attached might be confused in case they
find a pcie gpu which is *not* in a pcie root port (which is impossible
on physical hardware).

take care,
  Gerd