On Mon, Sep 10, 2012 at 07:50:13AM +0200, Paolo Bonzini wrote: > Il 09/09/2012 00:22, Michael S. Tsirkin ha scritto: > >> Almost. One is "the guest, if really needed, can tell the host of > >> pages". If not negotiated, and the host does not support it, the host > >> must break the guest (e.g. fail to offer any virtqueues). > > > > There is no way in spec to break the guest. > > You can not fail to offer virtqueues. > > You can always return 0 for the first queue. I don't think guest drivers recover gracefully from this. Do they? > > Besides, there is no guarantee that virtqueue setup > > happens after feature negotiation. > > It is the only way that makes sense though (unless the guest would write > 0 for its features). > Should we change that? Not sure. This was not always the case. Further setup can fail with e.g ENOMEM and driver could retry with a set of more conservative features. I do think it would be nice to add a generic way for device to notify guest about an internal failure. This can only happen after DRIVER_OK status is written though, and since existing drivers do not expect such failure, it might be too late. > >> The other is "the guest, though, would prefer not to do so". It is > >> different because the guest can proceed in a fallback mode even if the > >> host doesn't offer it. > > > > I think I get what your proposed SILENT means what I do not get > > is the motivation. It looks like a premature optimization to me. > > The motivation is to let the driver choose between two behaviors: the > current one where ballooning is only done on request, and a more > aggressive one. Yes but why is being silent any good? Optimization? Any data to show that it will help some workload? ... > > OK so TELL says *when* to notify host, SILENT if set allows guest > > to skip leak notifications? In this case TELL should just be ignored > > when SILENT is set. > > Yeah, that was my first idea. However, there are existing drivers that > ignore SILENT, so that would not be 100% exact. Not sure I follow the logic. They don't ack SILENT so that would be 100% exact. -- MST -- 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
