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Subject: Re: [virtio-comment] virtio and endian-ness
On Tue, 20 Aug 2013 14:04:10 +0930 Rusty Russell <rusty@au1.ibm.com> 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". 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. A feature bit would probably be easier for virtio-ccw - at least, there's a lot less to architecture on our side (no need for new channel commands etc.) > > 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. As they are just a means to do I/O as dictated by the hardware architecture, anything else would be weird. > > > Regarding little-endian rings, we could probably hack up some code to > > measure the overhead that would give us. > > That would be great! I'll follow up on this when we have some results.
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