On Thu, 2010-09-09 at 09:31 +0200, Gerd Hoffmann wrote: > > There is one special thing in the release ring usage though. As you > know qxl builds up a linked lists there, where the heads are in the > ring. qxl does also store the head of the list which it currently > builds into the ring, but doesn't bump prod (yet) so the guest will not > grab it. Which needs one extra ring entry. Because of that the IS_FULL > macro simply doesn't work for the release ring and the check is open coded. > > So, the old code checks whenever prod - cons + 1 != num_items. Lets > assume the guest didn't take out any entries yet, so cons is 0. When > prod is 7 the check fails and qxl will stop pushing stuff to the ring > due to being full. Lets have a look at the prod == 6 case: > > * The extra entry (ring[7%8]) holds the head for the list qxl > is building). > * qxl pushes that ring item to the guest (SPICE_RING_PUSH), > prod is 7 now. > * qxl gets a pointer to the next entry (SPICE_RING_PROD_ITEM), > which is ring[8%8]. > * qxl zero-initializes the ring entry as new list head, thereby > overriding ring[0], which isn't yet consumed by the guest. > > Correct?
No, that doesn't seem correct to me. Initially, both prod an cons are zero, and has num items = prod - cons = 0 - 0 = 0 Then we release an item (interface_release_resource). This starts writing to the current producer item SPICE_RING_PROD_ITEM(ring, item), either initializing it or appending to the list. In this case prod is zero, so we're writing to ring->items[0]. Then we push this, which increases prod to 1 and then initializes the new temp item (at 1). Eventually we reach the case where prod is 6 (and lets say cons is still 0). We put stuff in item 6, then SPICE_RING_PUSH increasing prod to 7, initialize item 7. No overwrite here. Then when prod is 7 we keep appending to the item 7 list, but never push it because prod - cons + 1 = 7 - 0 + 1 = 8 == num_items. _______________________________________________ Spice-devel mailing list Spice-devel@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/spice-devel
