virtio and endian-ness

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.


Thus what I'd like to suggest is a new field reporting to
guest whether device supports native format, little endian format,
or both.

This new field will naturally be only exposed in the new layout.


Thoughts?

-- 
MST