On 26/02/13 12:04, Michael S. Tsirkin wrote: > On Mon, Feb 25, 2013 at 04:27:45PM +0100, Cornelia Huck wrote: >> Here's the latest version of my patch series enabling ioeventfds >> on s390, again against kvm-next. >> >> Patches 1 and 2 (cleaning up initialization and exporting the virtio-ccw >> api) would make sense even independent of the ioeventfd enhancements. >> >> Patches 3-5 are concerned with adding a new type of ioeventfds for >> virtio-ccw notifications on s390. The naming is now hopefully clearer. >> We won't add ioeventfd support for the legacy s390-virtio transport. >> >> Please consider applying. > > I just had a thought: this makes us lookup the device on the bus > for each notification. It would be better to simply get the > device index from guest instead. > > We could validate that it matches the correct device, > if not - fallback to the current linear scan. > > We could return the index to guest for the next call. > > I know this needs guest changes but it's still not too late to > fix this for 3.9 guests so that we won't need to worry > about compatibility going forward. > Hmm, this would certainly have the best scalability, but such a change would require adotions to the virtio spec and getting this fully tested till 3.9 seems somewhat dangerous. So I would prefer to actually improve the lookup (e.g. with a tree or hash) in kvm_io_bus_write or something like that as a first step. We also have some guests in the wild (sles11sp3 beta) that already use the current interface, so compatibility is already a an interesting aspect (does being a beta counts?) Just thinking loud here, we might be able to do this in a compatible way. The vq number that we pass is 64bit, but we dont need such a big range. The guest could use the upper 32 bit as a cookie (old code will have 0 ---> list traversal). We must have a query cookie function, that will return 0 on older qemus, though. Such an optimization could be added later on without needing to rush. Ideas, opinions? Christian _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
