[PATCH v6 0/6] vhost: optimize enqueue

[Zhihong Wang <zhihong.wang@intel.com>]

This patch set optimizes the vhost enqueue function.

It implements the vhost logic from scratch into a single function designed
for high performance and good maintainability, and improves CPU efficiency
significantly by optimizing cache access, which means:

 *  Higher maximum throughput can be achieved for fast frontends like DPDK
    virtio pmd.

 *  Better scalability can be achieved that each vhost core can support
    more connections because it takes less cycles to handle each single
    frontend.

This patch set contains:

 1. A Windows VM compatibility fix for vhost enqueue in 16.07 release.

 2. A baseline patch to rewrite the vhost logic.

 3. A series of optimization patches added upon the baseline.

The main optimization techniques are:

 1. Reorder code to reduce CPU pipeline stall cycles.

 2. Batch update the used ring for better efficiency.

 3. Prefetch descriptor to hide cache latency.

 4. Remove useless volatile attribute to allow compiler optimization.

Code reordering and batch used ring update bring most of the performance
improvements.

In the existing code there're 2 callbacks for vhost enqueue:

 *  virtio_dev_merge_rx for mrg_rxbuf turned on cases.

 *  virtio_dev_rx for mrg_rxbuf turned off cases.

The performance of the existing code is not optimal, especially when the
mrg_rxbuf feature turned on. Besides, having 2 callback paths increases
maintenance efforts.

Also, there's a compatibility issue in the existing code which causes
Windows VM to hang when the mrg_rxbuf feature turned on.

---
Changes in v6:

 1. Merge duplicated code.

 2. Introduce a function for used ring write.

 3. Add necessary comments.

---
Changes in v5:

 1. Rebase to dpdk-next-virtio master.

 2. Rename variables to keep consistent in naming style.

 3. Small changes like return value adjustment and vertical alignment.

 4. Add details in commit log.

---
Changes in v4:

 1. Fix a Windows VM compatibility issue.

 2. Free shadow used ring in the right place.

 3. Add failure check for shadow used ring malloc.

 4. Refactor the code for clearer logic.

 5. Add PRINT_PACKET for debugging.

---
Changes in v3:

 1. Remove unnecessary memset which causes frontend stall on SNB & IVB.

 2. Rename variables to follow naming convention.

 3. Rewrite enqueue and delete the obsolete in the same patch.

---
Changes in v2:

 1. Split the big function into several small ones.

 2. Use multiple patches to explain each optimization.

 3. Add comments.

Zhihong Wang (6):
  vhost: fix windows vm hang
  vhost: rewrite enqueue
  vhost: remove useless volatile
  vhost: add desc prefetch
  vhost: batch update used ring
  vhost: optimize cache access

 lib/librte_vhost/vhost.c      |  20 +-
 lib/librte_vhost/vhost.h      |   6 +-
 lib/librte_vhost/vhost_user.c |  31 ++-
 lib/librte_vhost/virtio_net.c | 541 ++++++++++++++----------------------------
 4 files changed, 225 insertions(+), 373 deletions(-)

-- 
2.7.4