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

Anthony Liguori
2007-11-12T23:25:54Z

Dor Laor wrote:
> 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.

Hrm, I have to think about backwards compatibility at the virtio-pci
layer. virtio-pci basically exposes two things, the first is an ABI for
doing bidirectional notification and setting driver status. The second
is a standard and transparent mechanism for virt_rings.

I think that the first is simply enough that we don't need a feature
mask or a version number. Maybe perhaps with the status bits, I don't
know. For the virt_rings, if we had something more sophisticated than
virt_rings down the road, that can be discovered/configured in the
per-device configuration area so I don't think we need feature/version
info for that.

Rusty, Avi: any thoughts? I'm a bit on the fence.

Regards,

Anthony Liguori

> Regards,
> Dor