+ IXIA packet generator-> Guest VM 82599 VF port1 Rx burst-> Guest VM virtio
+ port 0 Tx burst-> tap -> Linux Bridge->82599 PF-> IXIA packet generator
+
+
+Virtio PMD Rx/Tx Callbacks
+--------------------------
+
+Virtio driver has 6 Rx callbacks and 3 Tx callbacks.
+
+Rx callbacks:
+
+#. ``virtio_recv_pkts``:
+ Regular version without mergeable Rx buffer support for split virtqueue.
+
+#. ``virtio_recv_mergeable_pkts``:
+ Regular version with mergeable Rx buffer support for split virtqueue.
+
+#. ``virtio_recv_pkts_vec``:
+ Vector version without mergeable Rx buffer support, also fixes the available
+ ring indexes and uses vector instructions to optimize performance for split
+ virtqueue.
+
+#. ``virtio_recv_pkts_inorder``:
+ In-order version with mergeable and non-mergeable Rx buffer support
+ for split virtqueue.
+
+#. ``virtio_recv_pkts_packed``:
+ Regular and in-order version without mergeable Rx buffer support for
+ packed virtqueue.
+
+#. ``virtio_recv_mergeable_pkts_packed``:
+ Regular and in-order version with mergeable Rx buffer support for packed
+ virtqueue.
+
+Tx callbacks:
+
+#. ``virtio_xmit_pkts``:
+ Regular version for split virtqueue.
+
+#. ``virtio_xmit_pkts_inorder``:
+ In-order version for split virtqueue.
+
+#. ``virtio_xmit_pkts_packed``:
+ Regular and in-order version for packed virtqueue.
+
+By default, the non-vector callbacks are used:
+
+* For Rx: If mergeable Rx buffers is disabled then ``virtio_recv_pkts``
+ or ``virtio_recv_pkts_packed`` will be used, otherwise
+ ``virtio_recv_mergeable_pkts`` or ``virtio_recv_mergeable_pkts_packed``
+ will be used.
+
+* For Tx: ``virtio_xmit_pkts`` or ``virtio_xmit_pkts_packed`` will be used.
+
+
+Vector callbacks will be used when:
+
+* Mergeable Rx buffers is disabled.
+
+The corresponding callbacks are:
+
+* For Rx: ``virtio_recv_pkts_vec``.
+
+There is no vector callbacks for packed virtqueue for now.
+
+
+Example of using the vector version of the virtio poll mode driver in
+``testpmd``::
+
+ testpmd -l 0-2 -n 4 -- -i --rxq=1 --txq=1 --nb-cores=1
+
+In-order callbacks only work on simulated virtio user vdev.
+
+For split virtqueue:
+
+* For Rx: If in-order is enabled then ``virtio_recv_pkts_inorder`` is used.
+
+* For Tx: If in-order is enabled then ``virtio_xmit_pkts_inorder`` is used.
+
+For packed virtqueue, the default callbacks already support the
+in-order feature.
+
+Interrupt mode
+--------------
+
+.. _virtio_interrupt_mode:
+
+There are three kinds of interrupts from a virtio device over PCI bus: config
+interrupt, Rx interrupts, and Tx interrupts. Config interrupt is used for
+notification of device configuration changes, especially link status (lsc).
+Interrupt mode is translated into Rx interrupts in the context of DPDK.
+
+.. Note::
+
+ Virtio PMD already has support for receiving lsc from qemu when the link
+ status changes, especially when vhost user disconnects. However, it fails
+ to do that if the VM is created by qemu 2.6.2 or below, since the
+ capability to detect vhost user disconnection is introduced in qemu 2.7.0.
+
+Prerequisites for Rx interrupts
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To support Rx interrupts,
+#. Check if guest kernel supports VFIO-NOIOMMU:
+
+ Linux started to support VFIO-NOIOMMU since 4.8.0. Make sure the guest
+ kernel is compiled with:
+
+ .. code-block:: console
+
+ CONFIG_VFIO_NOIOMMU=y
+
+#. Properly set msix vectors when starting VM:
+
+ Enable multi-queue when starting VM, and specify msix vectors in qemu
+ cmdline. (N+1) is the minimum, and (2N+2) is mostly recommended.
+
+ .. code-block:: console
+
+ $(QEMU) ... -device virtio-net-pci,mq=on,vectors=2N+2 ...
+
+#. In VM, insert vfio module in NOIOMMU mode:
+
+ .. code-block:: console
+
+ modprobe vfio enable_unsafe_noiommu_mode=1
+ modprobe vfio-pci
+
+#. In VM, bind the virtio device with vfio-pci:
+
+ .. code-block:: console
+
+ python usertools/dpdk-devbind.py -b vfio-pci 00:03.0
+
+Example
+~~~~~~~
+
+Here we use l3fwd-power as an example to show how to get started.
+
+ Example:
+
+ .. code-block:: console
+
+ $ l3fwd-power -l 0-1 -- -p 1 -P --config="(0,0,1)" \
+ --no-numa --parse-ptype
+
+
+Virtio PMD arguments
+--------------------
+
+Below devargs are supported by the PCI virtio driver:
+
+#. ``vdpa``:
+
+ A virtio device could also be driven by vDPA (vhost data path acceleration)
+ driver, and works as a HW vhost backend. This argument is used to specify
+ a virtio device needs to work in vDPA mode.
+ (Default: 0 (disabled))
+
+Below devargs are supported by the virtio-user vdev:
+
+#. ``path``:
+
+ It is used to specify a path to connect to vhost backend.
+
+#. ``mac``:
+
+ It is used to specify the MAC address.
+
+#. ``cq``:
+
+ It is used to enable the control queue. (Default: 0 (disabled))
+
+#. ``queue_size``:
+
+ It is used to specify the queue size. (Default: 256)
+
+#. ``queues``:
+
+ It is used to specify the queue number. (Default: 1)
+
+#. ``iface``:
+
+ It is used to specify the host interface name for vhost-kernel
+ backend.
+
+#. ``server``:
+
+ It is used to enable the server mode when using vhost-user backend.
+ (Default: 0 (disabled))
+
+#. ``mrg_rxbuf``:
+
+ It is used to enable virtio device mergeable Rx buffer feature.
+ (Default: 1 (enabled))
+
+#. ``in_order``:
+
+ It is used to enable virtio device in-order feature.
+ (Default: 1 (enabled))
+
+#. ``packed_vq``:
+
+ It is used to enable virtio device packed virtqueue feature.
+ (Default: 0 (disabled))