On Fri, Feb 22, 2019 at 10:47:03AM +0800, Yongji Xie wrote: > > > + > > > +To track inflight I/O, the queue region should be processed as follows: > > > + > > > +When receiving available buffers from the driver: > > > + > > > + 1. Get the next available head-descriptor index from available ring, > > > i > > > + > > > + 2. Set desc[i].inflight to 1 > > > + > > > +When supplying used buffers to the driver: > > > + > > > + 1. Get corresponding used head-descriptor index, i > > > + > > > + 2. Set desc[i].next to process_head > > > + > > > + 3. Set process_head to i > > > + > > > + 4. Steps 1,2,3 may be performed repeatedly if batching is possible > > > + > > > + 5. Increase the idx value of used ring by the size of the batch > > > + > > > + 6. Set the inflight field of each DescStateSplit entry in the batch > > > to 0 > > > + > > > + 7. Set used_idx to the idx value of used ring > > > + > > > +When reconnecting: > > > + > > > + 1. If the value of used_idx does not match the idx value of used > > > ring, > > > + > > > + (a) Subtract the value of used_idx from the idx value of used > > > ring to get > > > + the number of in-progress DescStateSplit entries > > > + > > > + (b) Set the inflight field of the in-progress DescStateSplit > > > entries which > > > + start from process_head to 0 > > > + > > > + (c) Set used_idx to the idx value of used ring > > > + > > > + 2. Resubmit each inflight DescStateSplit entry > > > > I re-read a couple of time and I still don't understand what it says. > > > > For simplicity consider split ring. So we want a list of heads that are > > outstanding. Fair enough. Now device finishes a head. What now? I needs > > to drop head from the list. But list is unidirectional (just next, no > > prev). So how can you drop an entry from the middle? > > > > The process_head is only used when slave crash between increasing the > idx value of used ring and updating used_idx. We use it to find the > in-progress DescStateSplit entries before the crash and complete them > when reconnecting. Make sure guest and slave have the same view for > inflight I/Os. >
But I don't understand how does the described process help do it? > In other case, the inflight field is enough to track inflight I/O. > When reconnecting, we go through all DescStateSplit entries and > re-submit the entry whose inflight field is equal to 1. What I don't understand is how do we know the order in which they have to be resubmitted. Reordering operations would be a big problem, won't it? Let's say I fetch descriptors A, B, C and start processing. how does memory look? Now I finished B and marked it used. How does memory look? I also wonder how do you address a crash between marking descriptor used and clearing inflight. Will you redo the descriptor? Is it always safe? What if it's a write?
