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Subject: Re: [virtio-comment] virtio and endian-ness
On Tue, Aug 20, 2013 at 10:23:13AM +0100, Pawel Moll wrote: > On Tue, 2013-08-20 at 05:34 +0100, Rusty Russell wrote: > > Yes, I think virtio-ccw and virtio-mmio could simply add a new feature > > bit to say "1.0 compliant". > > MMIO devices have "version" register. Currently it's 1. The OASIS spec > will bump it to 2 (main change will be the addressing scheme). So the > guest drivers may support both versions. > > Paweł No, updating revision is a way to disable compatibility in drivers. So old drivers currently do: if revision != 1 -> exit new drivers will do if revision < 1 || revision > 2 -> exit Now a host can disable loading old drivers by setting revision to 2. However this must not be required by spec, it must be possible to write a device that implements both old and new spec. I'll have to look at how mmio and ccw do feature bits to figure out if a feature bit will work for this. -- MST
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