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

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

Cornelia Huck <cornelia.huck@de.ibm.com> writes:
> On Mon, 19 Aug 2013 12:27:53 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
>> On Mon, Aug 19, 2013 at 11:09:44AM +0200, Cornelia Huck wrote:
>> > On Mon, 19 Aug 2013 12:08:35 +0300
>> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
>> > 
>> > > On Mon, Aug 19, 2013 at 05:17:30PM +0930, Rusty Russell wrote:
>> > > > "Michael S. Tsirkin" <mst@redhat.com> writes:
>> > > > > On Mon, Aug 19, 2013 at 12:21:03PM +0930, Rusty Russell wrote:
>> > > > >> "Michael S. Tsirkin" <mst@redhat.com> writes:
>> > > > >> > During the last TC meeting, we discussed making virtio little endian.
>> > > > >> > It was suggested that a feature bit can be used for this,
>> > > > >> > but I now think I see two problems:
>> > > > >> >
>> > > > >> >
>> > > > >> > 1. Features are optional,
>> > > > >> > in that there's no way for device to communicate to
>> > > > >> > guest that guest must ack a feature bit, and e.g. fail
>> > > > >> > if guest does not ack.
>> > > > >> >
>> > > > >> > On the other hand, it seems likely
>> > > > >> > that a hardware virtio device might want to *only* implement
>> > > > >> > little endian format and not both big and little endian.
>> > > > >> >
>> > > > >> > In other words this would be something Paolo once called
>> > > > >> > a "negative feature".
>> > > > >> >
>> > > > >> > 2. With virtio-pci we are running out of transport bits,
>> > > > >> > and need a new config space layout to add extra feature bits.
>
> Just as point of data: For virtio-ccw we should be able to easily
> extend feature bits.

Yes, I think virtio-ccw and virtio-mmio could simply add a new feature
bit to say "1.0 compliant".  In 20 years time that might look a
bit awkward, but that's probably OK unless there's an obvious
out-of-band mechanism to use.

BTW "standards compliant" for proposals so far means:
1) config fields are LE.
2) device-specfic headers are LE (eg. virtio_net_hdr).
3) fields in the ring are LE.

For virtio-ccw, the actual commands to manipulate the virtio devices and
bus (ie. struct ccw1) would remain BE, because that's the nature of CCW.

> Regarding little-endian rings, we could probably hack up some code to
> measure the overhead that would give us.

That would be great!

Thanks,
Rusty.