Re: [PATCH 2/6] virtio block driver for QEMU (v2)

[Dor Laor <dor.laor-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Anthony Liguori wrote:
> Dor Laor wrote:
>>>   
>> In general I think we need to add another feature or even version 
>> number ( I know you guys hate it).
>> The reason is - Let's say you dont change functionality but change 
>> the irq protocol (for example the isr won't be zeroed on read), then 
>> an old
>> guest driver wouldn't know it runs on a new host version and will 
>> have its irq line pulled up.
>> So I suggest adding a capability of VIRTIO_ISR_CLEAR_XXX or adding a 
>> version number.
>> Comments?
>
> I don't think we'll actually change the ISR protocol.  I think it's 
> the best that it can actually be.  However, if we do need to change 
> the ABI for some reason, I think the right thing to do is just use a 
> new device ID (since it's effectively a new device).
>
> Regards,
>
> Anthony Liguori
Changing the devid is acceptable and much more easier then backward 
compatibility support, I prefer it too.
Note that there are some disadvantages when changing the devid - For 
example,
if you installed an old device drivers in the guest kernel and after a 
while you bring the guest down,
upgrade the kvm host version and bring the guest back up.
If it has a new device id (and since the abi is not maintained the 
driver won't match VENDOR=virtio; DEVID=*),
then the guest won't have a driver for the new device.
In that case I think a guest agent can switch to fully virtualized 
device and afterwards install the driver automatically.
This is what we do in our production environment.
Regards,
Dor

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