[dpdk-dev] [PATCH v2] doc: clarify virtio PMD path selection

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

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

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

Signed-off-by: Wang Yinan <yinan.wang@intel.com>
---
 .../virtio_paths_selection_and_usage.rst      | 142 ++++++++++++++++++
 1 file changed, 142 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..f51741609
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@@ -0,0 +1,142 @@
+..  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 <https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/
+    virtio-v1.1-cs01.html#x1-700009>`_: With this feature negotiated, device
+    can receive larger packets by combining individual descriptors.
+*   `In-order <https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/
+    virtio-v1.1-cs01.html#x1-690008>`_: 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 <https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/
+    virtio-v1.1-cs01.html#x1-610007>`_: 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 now support three path of split virtqueue: Split virtqueue mergebale path,
+    Split virtqueue no-mergeable path, Split virtqueue vectorized RX path.
+*   Since qemu 4.2.0, Packed virtqueue mergeable path and Packed virtqueue non-mergeable
+    path can be supported.
+
+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