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Subject: Re: [virtio-comment] virtio and endian-ness
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? > CCW is currently native, ie. big > endian, since it's S/390. > > Anthony originally suggested two feature bits: a big endian and a little > endian bit. A device which acks neither needs a heuristic. This is > probably overkill given only CCW will have this problem. > > > Thus what I'd like to suggest is a new field reporting to > > guest whether device supports native format, little endian format, > > or both. > > > > This new field will naturally be only exposed in the new layout. > > I really don't want to support both endians; I'd rather have the current > endian hacks left for legacy virtio, where they're needed anyway. I > don't think legacy is worth fixing, either. > > Cheers, > Rusty.
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