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

[Cornelia Huck <cornelia.huck@de.ibm.com>]

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.
> > >> 
> > >> 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.
> 
> So, I'll focus on the config space for now.
> That's also mostly a PCI thing and clearly off datapath so we are safe
> from that POV (I think you mentioned some concerns on the last call).
> 
> Though we'll need to think about CCW.
> It could be benefitial to involve some folks who work on this:
> I think cornelia.huck@de.ibm.com and fred.konrad@greensocs.com
> were involved in the past.

I'm already here, just haven't time to respond yet :)

> 
> I'm not sure if we need to follow some procedure
> to invite people to get involved - could you
> handle that?
>