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Re: [libvirt] [PATCHv4 2/9] qemu: Record the default NIC model in the domain XML



On 03/26/2013 12:51 PM, Peter Krempa wrote:
> On 03/25/13 21:34, Laine Stump wrote:
>> On 03/15/2013 11:26 AM, Peter Krempa wrote:
>>> This patch implements the devices post parse cllback and uses it to
>>> fill
>>> the default qemu network card model into the XML if none is specified.
>>>
>>> Libvirt assumes that the network card model for qemu is the "rtl8139".
>>> Record this in the XML using the new callback to avoid user
>>> confusion.
>>
>> As I recall (from a previous Fedora-specific patch I had to make that
>> forced all "fedora-13" machinetypes in configs to be changed to
>> "pc-0.14"), just causing the parser to fill this in will not cause the
>> default value to actually be written to the config files. So when we get
>> to the point where we want to change the default, we won't have actually
>> addressed the issue of making sure that existing configs continue to use
>> rtl8139 rather than abruptly changing the guest's hardware at next boot
>> (or when migrated to another host).
>
> This isn't an issue in this case. The default is and always was to use
> the realtek card. We can't change that unfortunately for anything
> better :(. This is just to notify the user what is the actual model we
> are using in this case so I think it isn't that important to rewrite
> the XML file.
>
> In case it's missing in the config, it will be always parsed to the
> default value and when the config will be saved in the future it will
> be written. But the value will still be used.
>
>>
>> If we want to assure that existing guests have their default netdev
>> model written to config, we'll need to actually force the config to be
>> rewritten to disk. Take a look at the patch named
>
> I don't think this is really needed, but I will have a look if this is
> feasible in this case.

Actually, I think this *is* necessary, because part of the reason for
explicitly adding the model to the config even when it is the default is
that there is talk afloat to *change* libvirt's default interface model
from rtl8139 to e1000. When/if this happens, existing domains will need
to continue using the old default after upgrading libvirt and restarting
the guest. In order for that to work, the model needs to be written to
the persistent config on disk.



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