Re: [Qemu-devel] virtio balloon PCI class code

[Alexander Graf]

On 13.03.2012, at 04:30, David Gibson wrote:

> Currently the virtio-pci driver advertises the virtio balloon device
> as having the PCI class code PCI_CLASS_MEMORY_RAM.  Although the
> balloon device is vaguely related to RAM management, it doesn't
> operate anything like a PCI memory device in the sense of the class
> code (in fact it's all about taking RAM away from the guest, not
> obtaining it).  Nor does this PCI class code appear to be suggested by
> the virtio PCI specification.
> 
> This recently caused problems on the pseries machine - the class code
> caused the firmware to mark the corresponding device tree node as a
> memory node, which cause the guest to get horribly confused attempting
> to discover memory early in boot.  That was due to a guest kernel bug,
> but since the bug is widespread in existing deployed kernels, we don't
> really want to trigger it from qemu.
> 
> We can work around the problem in the guest firmware for now, but
> using PCI_CLASS_MEMORY_RAM for the balloon just seems wrong.  Are
> there things out there already that rely on this, or should we just
> drop the class code?

CC'ing Michael.


Alex