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+.. SPDX-License-Identifier: BSD-3-Clause
+ Copyright(c) 2010-2015 Intel Corporation.
Poll Mode Driver for Emulated Virtio NIC
========================================
Virtio is a para-virtualization framework initiated by IBM, and supported by KVM hypervisor.
In the Data Plane Development Kit (DPDK),
we provide a virtio Poll Mode Driver (PMD) as a software solution, comparing to SRIOV hardware solution,
-
for fast guest VM to guest VM communication and guest VM to host communication.
Vhost is a kernel acceleration module for virtio qemu backend.
Virtio Implementation in DPDK
-----------------------------
-For details about the virtio spec, refer to Virtio PCI Card Specification written by Rusty Russell.
+For details about the virtio spec, refer to the latest
+`VIRTIO (Virtual I/O) Device Specification
+<https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio>`_.
-As a PMD, virtio provides packet reception and transmission callbacks virtio_recv_pkts and virtio_xmit_pkts.
+As a PMD, virtio provides packet reception and transmission callbacks.
-In virtio_recv_pkts, index in range [vq->vq_used_cons_idx , vq->vq_ring.used->idx) in vring is available for virtio to burst out.
+In Rx, packets described by the used descriptors in vring are available
+for virtio to burst out.
-In virtio_xmit_pkts, same index range in vring is available for virtio to clean.
-Virtio will enqueue to be transmitted packets into vring, advance the vq->vq_ring.avail->idx,
-and then notify the host back end if necessary.
+In Tx, packets described by the used descriptors in vring are available
+for virtio to clean. Virtio will enqueue to be transmitted packets into
+vring, make them available to the device, and then notify the host back
+end if necessary.
Features and Limitations of virtio PMD
--------------------------------------
* The descriptor number for the Rx/Tx queue is hard-coded to be 256 by qemu 2.7 and below.
If given a different descriptor number by the upper application,
the virtio PMD generates a warning and fall back to the hard-coded value.
- Rx queue size can be configureable and up to 1024 since qemu 2.8 and above. Rx queue size is 256
+ Rx queue size can be configurable and up to 1024 since qemu 2.8 and above. Rx queue size is 256
by default. Tx queue size is still hard-coded to be 256.
* Features of mac/vlan filter are supported, negotiation with vhost/backend are needed to support them.
- When backend can't support vlan filter, virtio app on guest should disable vlan filter to make sure
- the virtio port is configured correctly. E.g. specify '--disable-hw-vlan' in testpmd command line.
+ When backend can't support vlan filter, virtio app on guest should not enable vlan filter in order
+ to make sure the virtio port is configured correctly. E.g. do not specify '--enable-hw-vlan' in testpmd
+ command line. Note that, mac/vlan filter is best effort: unwanted packets could still arrive.
* "RTE_PKTMBUF_HEADROOM" should be defined
no less than "sizeof(struct virtio_net_hdr_mrg_rxbuf)", which is 12 bytes when mergeable or
Virtio PMD Rx/Tx Callbacks
--------------------------
-Virtio driver has 3 Rx callbacks and 2 Tx callbacks.
+Virtio driver has 6 Rx callbacks and 3 Tx callbacks.
Rx callbacks:
#. ``virtio_recv_pkts``:
- Regular version without mergeable Rx buffer support.
+ Regular version without mergeable Rx buffer support for split virtqueue.
#. ``virtio_recv_mergeable_pkts``:
- Regular version with mergeable Rx buffer support.
+ 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.
+ 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.
+ Regular version for split virtqueue.
-#. ``virtio_xmit_pkts_simple``:
- Vector version fixes the available ring indexes to optimize performance.
+#. ``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`` is
- used; otherwise ``virtio_recv_mergeable_pkts``.
+* 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``.
+* For Tx: ``virtio_xmit_pkts`` or ``virtio_xmit_pkts_packed`` will be used.
Vector callbacks will be used when:
-* ``txq_flags`` is set to ``VIRTIO_SIMPLE_FLAGS`` (0xF01), which implies:
-
- * Single segment is specified.
-
- * No offload support is needed.
-
* Mergeable Rx buffers is disabled.
The corresponding callbacks are:
* For Rx: ``virtio_recv_pkts_vec``.
-* For Tx: ``virtio_xmit_pkts_simple``.
+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 --txqflags=0xF01 --rxq=1 --txq=1 --nb-cores=1
+ 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
--------------
$ 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))
+
+#. ``speed``:
+
+ It is used to specify link speed of virtio device. Link speed is a part of
+ link status structure. It could be requested by application using
+ rte_eth_link_get_nowait function.
+ (Default: 10000 (10G))
+
+#. ``vectorized``:
+
+ It is used to specify whether virtio device prefers to use vectorized path.
+ Afterwards, dependencies of vectorized path will be checked in path
+ election.
+ (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))
+
+#. ``speed``:
+
+ It is used to specify link speed of virtio device. Link speed is a part of
+ link status structure. It could be requested by application using
+ rte_eth_link_get_nowait function.
+ (Default: 10000 (10G))
+
+#. ``vectorized``:
+
+ It is used to specify whether virtio device prefers to use vectorized path.
+ Afterwards, dependencies of vectorized path will be checked in path
+ election.
+ (Default: 0 (disabled))
+
+Virtio paths Selection and Usage
+--------------------------------
+
+Logically virtio-PMD has 9 paths based on the combination of virtio features
+(Rx mergeable, In-order, Packed virtqueue), below is an introduction of these
+features:
+
+* `Rx mergeable <https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/
+ virtio-v1.1-cs01.html#x1-2140004>`_: With this feature negotiated, device
+ can receive large 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. With this feature negotiated,
+ driver will use descriptors in order.
+* `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 make it easier
+ for hardware to implement.
+
+Virtio paths Selection
+~~~~~~~~~~~~~~~~~~~~~~
+
+If packed virtqueue is not negotiated, below split virtqueue paths will 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 will be selected.
+#. Split virtqueue in-order mergeable path: If Rx mergeable and in-order feature are
+ both negotiated, this path will be selected.
+#. Split virtqueue in-order non-mergeable path: If in-order feature is negotiated and
+ Rx mergeable is not negotiated, this path will be selected.
+#. Split virtqueue vectorized Rx path: If Rx mergeable is disabled and no Rx offload
+ requested, this path will be selected.
+
+If packed virtqueue is negotiated, below packed virtqueue paths will 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, this path will be selected.
+#. Packed virtqueue in-order mergeable path: If in-order and Rx mergeable feature 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.
+#. Packed virtqueue vectorized Rx path: If building and running environment support
+ AVX512 && in-order feature is negotiated && Rx mergeable is not negotiated &&
+ TCP_LRO Rx offloading is disabled && vectorized option enabled,
+ this path will be selected.
+#. Packed virtqueue vectorized Tx path: If building and running environment support
+ AVX512 && in-order feature is negotiated && vectorized option enabled,
+ this path will be selected.
+
+Rx/Tx callbacks of each Virtio path
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Refer to above description, virtio path and corresponding Rx/Tx callbacks will
+be selected automatically. Rx callbacks and Tx callbacks for each virtio path
+are shown in below table:
+
+.. table:: Virtio Paths and Callbacks
+
+ ============================================ ================================= ========================
+ Virtio paths 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 non-meregable 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 non-mergeable path virtio_recv_pkts_packed virtio_xmit_pkts_packed
+ Packed virtqueue vectorized Rx path virtio_recv_pkts_packed_vec virtio_xmit_pkts_packed
+ Packed virtqueue vectorized Tx path virtio_recv_pkts_packed virtio_xmit_pkts_packed_vec
+ ============================================ ================================= ========================
+
+Virtio paths Support Status from Release to Release
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Virtio feature implementation:
+
+* In-order feature is supported since DPDK 18.08 by adding new Rx/Tx callbacks
+ ``virtio_recv_pkts_inorder`` and ``virtio_xmit_pkts_inorder``.
+* Packed virtqueue is supported since DPDK 19.02 by adding new Rx/Tx callbacks
+ ``virtio_recv_pkts_packed`` , ``virtio_recv_mergeable_pkts_packed`` and
+ ``virtio_xmit_pkts_packed``.
+
+All virtio paths support status are shown in below table:
+
+.. table:: Virtio Paths and Releases
+
+ ============================================ ============= ============= ============= =======
+ Virtio paths 16.11 ~ 18.05 18.08 ~ 18.11 19.02 ~ 19.11 20.05 ~
+ ============================================ ============= ============= ============= =======
+ Split virtqueue mergeable path Y Y Y Y
+ Split virtqueue non-mergeable path Y Y Y Y
+ Split virtqueue vectorized Rx path Y Y Y Y
+ Split virtqueue simple Tx path Y N N N
+ Split virtqueue in-order mergeable path Y Y Y
+ Split virtqueue in-order non-mergeable path Y Y Y
+ Packed virtqueue mergeable path Y Y
+ Packed virtqueue non-mergeable path Y Y
+ Packed virtqueue in-order mergeable path Y Y
+ Packed virtqueue in-order non-mergeable path Y Y
+ Packed virtqueue vectorized Rx path Y
+ Packed virtqueue vectorized Tx path Y
+ ============================================ ============= ============= ============= =======
+
+QEMU Support Status
+~~~~~~~~~~~~~~~~~~~
+
+* Qemu now supports three paths of split virtqueue: Split virtqueue mergeable path,
+ Split virtqueue non-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.
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