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

[David Gibson <dwg@au1.ibm.com>]

On Sun, Mar 18, 2012 at 02:38:42PM +0200, Michael S. Tsirkin wrote:
> On Fri, Mar 16, 2012 at 12:03:08PM +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>
> 
> Well, this gives the class a legacy value 00,
> and the PCI spec says we should not use this:
> 
> D.1.
> Base Class 00h
> This base class is defined to provide backward compatibility for devices
> that were built
> before the Class Code field was defined. No new devices should use this
> value and existing
> devices should switch to a more appropriate value if possible.
> For class codes with this base class value, there are two defined values
> for the remaining
> fields as shown in the table below. All other values are reserved.
> Base Class
> 00h
> Sub-Class
> Interface
> 00h
> 01h
> VGA-compatible device
> 00h
> 00h
> All currently implemented devices
> except VGA-compatible devices
> 
> You probably want this instead:
> 
> #define PCI_CLASS_OTHERS		0xff

Ah, thanks.  I had mistakenly assumed that 0 meant "no class
specified".  I'll respin changing to PCI_CLASS_OTHERS.

-- 
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