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Subject: Re: [virtio-comment] virtio and endian-ness
"Michael S. Tsirkin" <firstname.lastname@example.org> writes: > On Mon, Aug 19, 2013 at 12:21:03PM +0930, Rusty Russell wrote: >> "Michael S. Tsirkin" <email@example.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.