On Fri, Sep 07, 2012 at 02:22:47PM +0200, Paolo Bonzini wrote: > Il 07/09/2012 14:17, Michael S. Tsirkin ha scritto: > >>> Next, consider the interface proposed here. You defacto declare > >>> all existing drivers buggy. > >> > >> No, only Windows (and it is buggy, it calls tell_host last). > > > > It is not buggy. It does not ack MUST_TELL_HOST. So it is free to tell > > host at any point, it behaves exactly > > to spec. You can not retroactively declare drivers buggy like that. > > Well, according to your reading of the spec. > > In the spec I'm reading "Host must be told before pages from the balloon > are used". Doesn't say anything about the guest. No? How is host told then? By divine force? > Now, indeed a very free interpretation is "Guest will tell host before > pages from the balloon are used". It turns out that it's exactly what > guests have been doing, hence that's exactly what I'm proposing now: > rename the feature to VIRTIO_BALLOON_F_WILL_TELL_HOST. Rename is fine. > >> True, but the choice is: > >> > >> 1) add a once-only hack to QEMU that fixes migration of > >> VIRTIO_BALLOON_F_MUST_TELL_HOST; > >> > >> 2) always advertise VIRTIO_BALLOON_F_MUST_TELL_HOST. If you do this, > >> guests cannot use anymore silent deflate, which is a regression. > >> > >> 3) use two bits. One tells the device that the driver supports chatty > >> deflate; one tells the driver that the device supports silent deflate. > > > > The right thing to do is either > > 4. realize we can not address all user errors, so no real issue > > 5. address this class of user errors by migrating host features > > > >> So in the end you do use two feature bits for two different things. > >> Plus, both feature bits are "positive" and I'm happy. > > > > I am not happy. > > We lose compatibility with all existing drivers > > How so? > > > so it will take years until the feature is actually > > useful. > > No, we don't! Windows guests will just not be able to use munlock/mlock > until the driver is fixed. Which will probably happen before someone > writes the munlock/mlock code. If the only change is rename then ofcourse things keep working. I don't care about the name it is up to Rusty. > > Is this just a matter of naming? Same functionality: > > driver that acks this bit will tell host first, > > driver that does not will not? > > > > If yes that is fine. > > Yes, that part we agree on I think. We disagree that (after the rename) > QEMU should start always proposing VIRTIO_BALLOON_F_WILL_TELL_HOST. Not always. It must be off if compatibility with old qemu is disabled. > _Plus_ adding the new feature bit, which is needed to actually tell the This part I do not get. What is silent deflate, why is it useful and what it has to do with what we are discussing here? > driver that the host supports the silent deflate. > Spec patch on the way. > > Paolo Hang on. Can we please talk about motivation? These patches which come without motivation are very hard to review. -- 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
