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

["Gavin Hu (Arm Technology China)" <Gavin.Hu@arm.com>]

Hi Yinan,

> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Yinan
> Sent: Monday, November 18, 2019 6:29 PM
> To: dev@dpdk.org; maxime.coquelin@redhat.com; tiwei.bie@intel.com
> Cc: zhihong.wang@intel.com; john.mcnamara@intel.com;
> marko.kovacevic@intel.com; Yinan Wang <yinan.wang@intel.com>
> Subject: [dpdk-dev] [PATCH] doc: clarify virtio PMD path selection
> 
> 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
> --- /dev/null
> +++ b/doc/guides/howto/virtio_paths_selection_and_usage.rst
> @@ -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:
s/are/is an
> +
> +*   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
> +-------------------
It is better to add Qemu version latest, the future Qemu version may support more.
> +
> +Qemu only support three path of Virtio-PMD: Split virtqueue mergebale path,
Add "s" to the first 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

Other than the minor comments, 
Reviewed-by: Gavin Hu <gavin.hu@arm.com>