On Wed, Nov 16, 2011 at 10:28:05AM +1030, Rusty Russell wrote: > On Sun, 13 Nov 2011 17:14:27 +0200, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > On Fri, Nov 11, 2011 at 02:54:31PM +1030, Rusty Russell wrote: > > > Indeed, I'd like to see two changes to your proposal: > > > > > > (1) It should be all or nothing. If a driver can find the virtio header > > > capability, it should only use the capabilties. Otherwise, it > > > should fall back to legacy. > > > > Okay, but going forward, if we add more capabilities, we probably won't > > want to require them and fail to load if not there. That's really why I > > wanted to make the failover ignore any capability separately - to make > > this future proof. I'm not terribly fixated on this, it just seemed a > > bit more symmetrical to treat all capabilities in the same way. Hmm? > > Sure, a future capbility may not exist. But once a driver finds that > virtio header structure in the capability, it should *never* fall back > to the legacy area. ie. it can expect that Queue Notify, ISR Status and > Device Header all exist. > > ie. either use legacy mode, or use capabilities. Never both. > > > > > > Your draft suggests a mix is possible; > > > I prefer a clean failure (ie. one day don't present a BAR 0 *at > > > all*, so ancient drivers just fail to load.). > > > > Just to clarify, as written in draft this is possible with the current > > spec proposal. So I'm guessing there's some other motivation that you > > had in mind? > > At the moment you give a hybrid model where both are used. In five > years' time, that's going to be particularly ugly. > > > > > (2) There's no huge win in keeping the same layout. Let's make some > > > cleanups. > > > > About this last point - what cleanups do you have in mind? Just move > > some registers around? I guess we could put feature bits near each > > other, and move device status towards the end to avoid wasting 3 > > bytes. > > > The win seems minimal, but the change does make legacy hypervisor > > support in guests more cumbersome, as we need to spread coditional code > > around instead of localizing it in the initialization path. > > But the separation between "legacy" and "modern" will be sharper, making > it easier to excise the legacy portion later. > > And in five years' time, people implementing virtio will really thank us > that they can completely ignore the legacy header. OK, I get it I think. > > > There are more users ahead of us then behind us (I > > > hope!). > > > > In that case isn't it safe to assume we'll find some uses > > for the reserved registers? > > How would we tell? If we use a new capability struct for it, it's > obvious. Otherwise, you're going to need to steal more feature bits. Yes, exactly, if as you suggest, we disable legacy header when there is a capability - we can use reserved registers for other stuff. > Cheers, > Rusty. > PS. Sorry, was off sick for a few days. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
