On Tue, Mar 20, 2012 at 09:19:47PM +1100, David Gibson wrote: > On Tue, Mar 20, 2012 at 09:54:20AM +0000, Stefan Hajnoczi wrote: > > On Tue, Mar 20, 2012 at 12:42 AM, David Gibson > > <da...@gibson.dropbear.id.au> wrote: > > > On Mon, Mar 19, 2012 at 11:33:10AM +0000, Stefan Hajnoczi wrote: > > >> On Mon, Mar 19, 2012 at 03:59:23PM +1100, David Gibson wrote: > > >> > Currently the virtio balloon device, when using the virtio-pci > > >> > interface > > >> > advertises itself with PCI class code MEMORY_RAM. This is wrong; the > > >> > balloon is vaguely related to memory, but is nothing like a PCI memory > > >> > device in the meaning of the class code, and this code is not required > > >> > or > > >> > suggested by the virtio PCI specification. > > >> > > > >> > Worse, this patch causes problems on the pseries machine, because the > > >> > firmware, seeing this class code, advertises the device as memory in > > >> > the > > >> > device tree, and then a guest kernel bug causes it to see this "memory" > > >> > before the real system memory, leading to a crash in early boot. > > >> > > > >> > This patch fixes the problem by removing the bogus PCI class code on > > >> > the > > >> > balloon device. > > >> > > > >> > Cc: Michael S. Tsirkin <m...@redhat.com> > > >> > Cc: Rusty Russell <ru...@rustcorp.com.au> > > >> > > > >> > Signed-off-by: David Gibson <da...@gibson.dropbear.id.au> > > >> > --- > > >> > hw/virtio-pci.c | 2 +- > > >> > 1 files changed, 1 insertions(+), 1 deletions(-) > > >> > > >> Since this is a guest-visible change we might need to be careful about > > >> how it's introduced. > > >> > > >> Do we need to keep the old class code for existing machine types? The > > >> new class code could be introduced only for 1.1 and later machine types > > >> if we want to be extra careful about introducing guest-visible > > >> changes. > > > > > > So as a general rule, I like to be very careful about user-visible > > > changes. But in this case, I don't think we want to be too hesitant. > > > In particular, it's not just a question of the machine type, but also > > > of how the guest OS will deal with the PCI class code. > > > > > > The class code we were using was Just Plain Wrong. It was not > > > suggetsed by the virtio spec, and it makes no sense. It happens that > > > so far this caused problems only for a guest on a particular machine > > > type, but there's no reason it couldn't cause (different) problems for > > > guests on any machine type. > > > > > > More to the point, it seems reasonably unlikely for existing guests to > > > rely on the broken behaviour: again, there's no reason they'd think > > > they need to based on the spec, and the usual way of matching drivers > > > to PCI devices is with the vendor/device IDs which are correct and not > > > changed by this patch. > > > > > > So, unless we have a known example of an existing guest that would be > > > broken by this change, I think we should implement it ASAP for all > > > machine types. > > > > I agree that in practice the risk is low because working guests are > > probably not using the class code. On the other hand I don't see a > > downside to making this part of the 1.1 machine type, > > Well.. there's the fact that I can't what mechanism we would use to > make this per-machine...
Not sure I parsed this correctly, but I think you're asking how to do it. Looking at hw/pc_piix.c there are QEMUMachine types for each QEMU release. Legacy machine types (e.g. pc_machine_v0_14) have a .compat_props array that can override qdev properties. Perhaps Michael Tsirkin or someone else can comment on how to wire up hw/virtio-pci.c so that the class code can be overridden. Stefan
