Re: [virtio-comment] virtio and endian-ness

[Rusty Russell <rusty@au1.ibm.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:
> On Tue, Aug 20, 2013 at 04:14:09PM +0100, Pawel Moll wrote:
>> On Tue, 2013-08-20 at 16:08 +0100, Michael S. Tsirkin wrote:
>> > Confused.
>> > You want to support old and new devices in the same
>> > driver, don't you? Isn't that making it more complicated?
>> 
>> It will make the driver more complicated, yes (slightly, as the *MMIO*
>> changes are not going to be too extensive). But to my knowledge only one
>> virtio_mmio driver has been ever implemented (the Linux one), where I
>> can do the right thing quite easily and this is a common pattern for the
>> kernel drivers (a single one can driver a family of almost, but not
>> quite, identical devices). I just don't want to make life harder for the
>> guys implementing the device component (I'll say this again: virtio_mmio
>> interface was conceived to be as simple as possible :-) in qemu, kvm
>> tools, proprietary modelling environments, <pick your choice here>.
>> 
>> Paweł
>
> Right.
>
> I think spec should give devices the choice.
>
> If they don't want old drivers to work,
> they can bump version number.
> If they want old drivers to work, don't
> bump version number.
>
> This implies that another mechanism (besides version number)
> is used by driver and device to detect new spec.
>
> Actually feature bit starts looking OK to me
> as long as it doesn't mean "v2 driver required",
> instead meaning "v2 driver supported".
> We specify that v2 driver always acks this bit.

Yes, precisely.

> So if you (like Paweł) don't want to support old drivers,
> just bump revision and set this bit.
> Then if driver does not ack this bit -> it's a driver bug.
>
> If you want to support old drivers - don't bump
> revision, and set this feature bit.
> You can look at the feature bit to distinguish old
> drivers from new ones.

That scheme would have ultimate flexibility, but I'm not sure
that it's any better in practice than just giving up if you only
implement "standard" virtio and the driver doesn't ack the bit.

Cheers,
Rusty.