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

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

"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.
>> 
>> The discussion was more in the context of a method for backwards
>> compatiblity.  If we're changing the PCI layout, that itself is
>> sufficient to trigger LE-only mode.
>>
>> MMIO is already defined as LE-only.
>
> Sorry, are we talking about only config layout
> for now, or ring and headers in guest memory as well?
>
> The later is native for MMIO, isn't it?

Sorry, you're right.

I was talking about everything, though they're potentially separate
decisions.

Cheers,
Rusty.