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

[David Gibson <david@gibson.dropbear.id.au>]

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...

> which is what
> users will run when they get this code change anyway.  That way we can
> tell users that we never change the device model in a release with a
> straight face :).
> 
> Anthony: I'm not sure how strict we are about a user-visible change like this?
> 
> Stefan
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson