On Thu, 2012-01-12 at 08:09 +0200, Michael S. Tsirkin wrote: > On Thu, Jan 12, 2012 at 03:31:59PM +1100, Benjamin Herrenschmidt wrote: > > However I can see at least one advantage of what you've done :-) You > > never have to deal with holes in the ring. > > Another advantage is the design goal for that ring: > host never needs to copy even if it completes > descriptors out of order. And out of order is something that does not > happen at all with hardware drivers. This is where paravirt is > different. Actually out of order can happen with tagged command queue for SCSI or ATA, tho I'm not 100% familiar with how things like AHCI handle this. > > > > Two rings do have the advantage of not requiring host side copy, which > > > > copy would surely add to cache pressure. > > > > > > Well, a simple host could process in-order and leave stuff in the ring I > > > guess. A smarter host would copy and queue, maybe leave one queue entry > > > in so it doesn't get flooded? > > > > What's wrong with a ring of descriptors + a ring of completion, with a > > single toggle valid bit to indicate whether a given descriptor is valid > > or not (to avoid the nasty ping pong on the ring head/tails). > > First, I don't understand how a valid bit avoids ping poing on the last > descriptor. Second, how do you handle out of order completions? A toggle means the valid bit is never cleared, it just changes polarity every time you go around the ring. So there's never a write back to 0. Out of order is something I hadn't thought about (I was most probably too focused on virtio-net) and is indeed a PITA. It's doable with rings but can get nasty. I'll give that more thought in the next week, and Rusty and I shall play with userspace models based on your tool. > > > > About inline - it can only help very small buffers. > > > > Which workloads do you have in mind exactly? > > > > > > It was suggested by others, but I think TCP Acks are the classic one. > > > > Split headers + data too, tho that means supporting immediate + > > indirect. > > > > It makes a lot of sense for command rings as well if we're going to go > > down that route. > > I don't see why it makes sense for commands. It's a performance > optimization and commands are off the data path. Oh just code simplification not having to dequeue a descriptor, allocate a buffer, etc... but a lot of that can be buried in helpers indeed. > We can't, legal PCI ProgIf values are defined in PCI spec. Hrm, more or less yes, I suppose we should stay away from that then, do we use revision ID for anything in virtio-land ? Cheers, Ben. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
