[dpdk-dev] [PATCH] doc: clarify virtio PMD path selection

[Yinan <yinan.wang@intel.com>]

From: Yinan Wang <yinan.wang@intel.com>

add virtio paths selection and usage introduction for better
virtio usability.

Signed-off-by: Yinan Wang <yinan.wang@intel.com>
---
 .../virtio_paths_selection_and_usage.rst      | 130 ++++++++++++++++++
 1 file changed, 130 insertions(+)
 create mode 100644 doc/guides/howto/virtio_paths_selection_and_usage.rst

diff --git a/doc/guides/howto/virtio_paths_selection_and_usage.rst b/doc/guides/howto/virtio_paths_selection_and_usage.rst
new file mode 100644
index 000000000..b5918eaf9
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@@ -0,0 +1,130 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2019 Intel Corporation.
+
+Virtio paths Selection and Usage
+================================
+
+Logically virtio-PMD has 9 paths based on the virtio features (Rx mergeable,In-order,Packed virtqueue)
+combinations, below are introduction of virtio three common features:
+
+*   Rx mergeable: With this feature negotiated, device can receive larger packets by combining
+    individual descriptors.
+*   In-order: Some devices always use descriptors in the same order in which they have been made
+    available, these devices can offer the VIRTIO_F_IN_ORDER feature. If this feature negotiated,
+    driver will use descriptors in order. Meanwhile, this knowledge allows device operate used ring
+    in batches and driver operate available ring in batches and such can decrease cache miss rate.
+*   Packed virtqueue: The structure of packed virtqueue is different from split virtqueue，split
+    virtqueue is composed of available ring, used ring and descriptor table, while packed virtqueue
+    is composed of descriptor ring，driver event suppression and device event suppression. The idea
+    behind this is to improve performance by avoiding cache misses and and make it easier for devices
+    to implement.
+
+Virtio paths Selection
+----------------------
+
+If packed virtqueue is not negotiated, below split virtqueue paths can be selected
+according to below configuration:
+
+#. Split virtqueue mergeable path: If Rx mergeable is negotiated, in-order feature is
+   not negotiated, this path will be selected.
+#. Split virtqueue non-mergeable path: If Rx mergeable and in-order feature are not
+   negotiated, also Rx offload(s) are requested, this path can be selected.
+#. Split virtqueue in-order mergeable path: If in-order feature and Rx mergeable are
+   both negotiated, this path can be selected.
+#. Split virtqueue in-order non-mergeable path: If in-order feature is negotiated and
+   Rx mergeable is not negotiated, this path can be selected.
+#. Split virtqueue vectorized RX path: If Rx mergeable is disabled and no Rx offload
+   requested, this path can be selected.
+
+If packed virtqueue is negotiated, below packed virtqueue paths can be selected
+according to below configuration:
+
+#. Packed virtqueue mergeable path: If Rx mergeable is negotiated, in-order feature
+   is not negotiated, this path will be selected.
+#. Packed virtqueue non-mergeable path: If Rx mergeable and in-order feature are not
+   negotiated, also Rx offload(s) are requested, this path will be selected.
+#. Packed virtqueue in-order mergeable path: If in-order feature and Rx mergeable are
+   both negotiated, this path will be selected.
+#. Packed virtqueue in-order non-mergeable path: If in-order feature is negotiated and
+   Rx mergeable is not negotiated, this path will be selected.
+
+Rx/Tx callbacks of each Virtio path
+-----------------------------------
+
+Refer to above descriptions,virtio path and Rx/TX callbacks are auto selected by different parameters of
+vdev and workloads. Rx callbacks and Tx callbacks name for each Virtio Path are shown in following tables::
+
+   +----------------------------------------------------------------------------------------------------------+
+   |       Virtio path                          |   Rx callbacks                   |    TX callbacks          |
+   +----------------------------------------------------------------------------------------------------------+
+   |Split virtqueue mergeable path              |virtio_recv_mergeable_pkts        | virtio_xmit_pkts         |
+   +----------------------------------------------------------------------------------------------------------+
+   |Split virtqueue non-mergeable path          | virtio_recv_pkts                 |  virtio_xmit_pkts        |
+   +----------------------------------------------------------------------------------------------------------+
+   |Split virtqueue in-order mergeable path     | virtio_recv_pkts_inorder         |  virtio_xmit_pkts_inorder|
+   +----------------------------------------------------------------------------------------------------------+
+   |Split virtqueue in-order non-mergeable path | virtio_recv_pkts_inorder         |  virtio_xmit_pkts_inorder|
+   +----------------------------------------------------------------------------------------------------------+
+   |Split virtqueue vectorized RX path          | virtio_recv_pkts_vec             |  virtio_xmit_pkts        |
+   +----------------------------------------------------------------------------------------------------------+
+   |Packed virtqueue mergeable path             | virtio_recv_mergeable_pkts_packed|  virtio_xmit_pkts_packed |
+   +----------------------------------------------------------------------------------------------------------+
+   |Packed virtqueue normal path                | virtio_recv_pkts_packed          |  virtio_xmit_pkts_packed |
+   +----------------------------------------------------------------------------------------------------------+
+   |Packed virtqueue in-order mergeable path    | virtio_recv_mergeable_pkts_packed|  virtio_xmit_pkts_packed |
+   +----------------------------------------------------------------------------------------------------------+
+   |Packed virtqueue in-order normal path       | virtio_recv_pkts_packed          |  virtio_xmit_pkts_packed |
+   +----------------------------------------------------------------------------------------------------------+
+
+Virtio paths Support Status from Release to Release
+---------------------------------------------------
+
+Virtio feature implementation:
+
+*   In-order feature implemented in DPDK 18.08 by adding new Rx/TX callbacks
+    ``virtio_recv_pkts_inorder`` and ``virtio_xmit_pkts_inorder``.
+*   Packed virtqueue implemented in DPDK 19.02 by adding new Rx/TX callbacks
+    ``virtio_recv_pkts_packed`` , ``virtio_recv_mergeable_pkts_packed`` and ``virtio_xmit_pkts_packed``.
+
+Virtio path number changes from release to release, all virtio paths support status are shown in below table::
+
+   +--------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Virtio path\ DPDK version                   | v16.11 | v17.02 | v17.05 | v17.08 | v17.11 | v18.02 | v18.05 | v18.08 | v18.11 | v19.02 | v19.05 | v19.08 |
+   +--------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Split virtqueue mergebale path              |   Y    |   Y    |    Y    |   Y   |   Y    |   Y    |   Y    |   Y    |   Y    |  Y     |   Y    |   Y    |
+   +--------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Split virtqueue non-mergeable path          |   Y    |   Y    |    Y    |   Y   |   Y    |   Y    |   Y    |   Y    |   Y    |  Y     |   Y    |   Y    |
+   +--------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Split virtqueue vectorized RX path          |   Y    |   Y    |    Y    |   Y   |   Y    |   Y    |   Y    |   Y    |   Y    |  Y     |   Y    |   Y    |
+   ---------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Split virtqueue simple TX path              |   Y    |   Y    |    Y    |   Y   |   Y    |   Y    |   Y    |   N    |   N    |  N     |   N    |   N    |
+   ---------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Split virtqueue in-order non-mergeable path |        |        |         |       |        |        |        |   Y    |   Y    |  Y     |   Y    |   Y    |
+   ---------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Split virtqueue in-order mergeable path     |        |        |         |       |        |        |        |   Y    |   Y    |  Y     |   Y    |   Y    |
+   ---------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Packed virtqueue mergeable path             |        |        |         |       |        |        |        |        |        |  Y     |   Y    |   Y    |
+   ---------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Packed virtqueue non-mergeable path         |        |        |         |       |        |        |        |        |        |  Y     |   Y    |   Y    |
+   ---------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Packed virtqueue in-order mergeable path    |        |        |         |       |        |        |        |        |        |  Y     |   Y    |   Y    |
+   ---------------------------------------------------------------------------------------------------------------------------------------------------------+
+   |Packed virtqueue in-order non-mergeable path|        |        |         |       |        |        |        |        |        |  Y     |   Y    |   Y    |
+   ---------------------------------------------------------------------------------------------------------------------------------------------------------+
+
+QEMU Support Status
+-------------------
+
+Qemu only support three path of Virtio-PMD: Split virtqueue mergebale path,
+Split virtqueue no-mergeable path，Split virtqueue vectorized RX path.
+
+How to Debug
+------------
+
+If you meet performance drop or some other issues after upgrading the driver
+or configuration, below steps can help you identify which path you selected and
+root cause faster.
+
+#. Run vhost/virtio test case;
+#. Run "perf top" and check virtio Rx/tx callback names;
+#. Identify which virtio path is selected refer to above table.
-- 
2.17.1