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Subject: Re: [virtio-comment] virtio and endian-ness
Il 20/08/2013 10:39, Michael S. Tsirkin ha scritto: > On Tue, Aug 20, 2013 at 02:04:10PM +0930, Rusty Russell 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". > > Again, this would be a "negative feature", won't it? Yes, unless the device model implements both pre-standard and post-standard layouts. Perhaps we can reserve the high 16 bits of the device ID for the standard revision? Thus pre-standard layout would have device IDs like 0x0000xxxx (as it is now). Post-standard layout would be 0x0001xxxx. Paolo
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