Il 17/07/2012 12:49, Michael S. Tsirkin ha scritto: >> Ok, that would make more sense. One difference between vhost-blk and >> vhost-net is that for vhost-blk there are also management actions that >> would trigger the switch, for example a live snapshot. >> So a prerequisite for vhost-blk would be that it is possible to disable >> it on the fly while the VM is running, as soon as all in-flight I/O is >> completed. > > It applies for vhost-net too. For example if you bring link down, > we switch to userspace. So vhost-net supports this switch on the fly. Cool. >> (Note that, however, this is not possible for vhost-scsi, because it >> really exposes different hardware to the guest. It must not happen that >> a kernel upgrade or downgrade toggles between userspace SCSI and >> vhost-scsi, for example). > > I would say this is not a prerequisite for merging in qemu. > It might be a required feature for production but it > is also solvable at the management level. I'm thinking of the level interrupts here. You cannot make a change in the guest, and have it do completely unrelated changes the hardware that the guest sees. >>>> having to >>>> support the API; having to handle transition from one more thing when >>>> something better comes out. >>> >>> Well this is true for any code. If the limited featureset which >>> vhost-blk can accelerate is something many people use, then accelerating >>> by 5-15% might outweight support costs. >> >> It is definitely what people use if they are interested in performance. > > In that case it seems to me we should stop using the feature set as > an argument and focus on whether the extra code is worth the 5-15% gain. > No one seems to have commented on that so everyone on list thinks that > aspect is OK? I would like to see a breakdown of _where_ the 5-15% lies, something like http://www.linux-kvm.org/page/Virtio/Block/Latency. > Kernel merge windows is coming up and I would like to see whether > any of vhost-blk / vhost-scsi is going to be actually used by userspace. > I guess we could tag it for staging but would be nice to avoid that. Staging would be fine by me for both vhost-blk and vhost-scsi. Paolo -- 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
