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Subject: Re: [virtio-comment] virtio and endian-ness
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? >
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