Re: [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon device

[Stefan Hajnoczi <stefanha@gmail.com>]

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
> > <david@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 <mst@redhat.com>
> > >> > Cc: Rusty Russell <rusty@rustcorp.com.au>
> > >> >
> > >> > Signed-off-by: David Gibson <david@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