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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 20, 2013 at 02:04:10PM +0930, Rusty Russell wrote:
> 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".

Again, this would be a "negative feature", won't it?

> 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.