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Date: Wed, 21 Mar 2012 09:42:41 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] Remove PCI class code from virtio balloon
 device
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: qemu-trivial@nongnu.org, Stefan Hajnoczi <stefanha@gmail.com>, Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org

On 03/21/2012 08:08 AM, Michael S. Tsirkin wrote:
> On Wed, Mar 21, 2012 at 11:26:15AM +0000, Stefan Hajnoczi wrote:
>> On Tue, Mar 20, 2012 at 09:19:47PM +1100, David Gibson wrote:
>> 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
>
> afaik we already let users over-write it for some other pci devices,
> look there for examples.

 From hw/pc_piix.c:

     .name = "pc-0.10",
     .desc = "Standard PC, qemu 0.10",
     .init = pc_init_pci_no_kvmclock,
     .max_cpus = 255,
     .compat_props = (GlobalProperty[]) {
         {
             .driver   = "virtio-blk-pci",
             .property = "class",
             .value    = stringify(PCI_CLASS_STORAGE_OTHER),
         },{

And from the earlier part of the thread, yes, it's imperative that we do not 
change anything in the PCI configuration space for older pc versions regardless 
of whether it may or may not work.

Certain guests (like Windows) use a complex fingerprinting algorithm to 
determine when hardware changes.  It can be hard to detect in simple testing 
because it's based on a threshold.

Regards,

Anthony Liguori

>
>