netvsc.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3======================
  4Hyper-V network driver
  5======================
  6
  7Compatibility
  8=============
  9
 10This driver is compatible with Windows Server 2012 R2, 2016 and
 11Windows 10.
 12
 13Features
 14========
 15
 16Checksum offload
 17----------------
 18  The netvsc driver supports checksum offload as long as the
 19  Hyper-V host version does. Windows Server 2016 and Azure
 20  support checksum offload for TCP and UDP for both IPv4 and
 21  IPv6. Windows Server 2012 only supports checksum offload for TCP.
 22
 23Receive Side Scaling
 24--------------------
 25  Hyper-V supports receive side scaling. For TCP & UDP, packets can
 26  be distributed among available queues based on IP address and port
 27  number.
 28
 29  For TCP & UDP, we can switch hash level between L3 and L4 by ethtool
 30  command. TCP/UDP over IPv4 and v6 can be set differently. The default
 31  hash level is L4. We currently only allow switching TX hash level
 32  from within the guests.
 33
 34  On Azure, fragmented UDP packets have high loss rate with L4
 35  hashing. Using L3 hashing is recommended in this case.
 36
 37  For example, for UDP over IPv4 on eth0:
 38
 39  To include UDP port numbers in hashing::
 40
 41	ethtool -N eth0 rx-flow-hash udp4 sdfn
 42
 43  To exclude UDP port numbers in hashing::
 44
 45	ethtool -N eth0 rx-flow-hash udp4 sd
 46
 47  To show UDP hash level::
 48
 49	ethtool -n eth0 rx-flow-hash udp4
 50
 51Generic Receive Offload, aka GRO
 52--------------------------------
 53  The driver supports GRO and it is enabled by default. GRO coalesces
 54  like packets and significantly reduces CPU usage under heavy Rx
 55  load.
 56
 57Large Receive Offload (LRO), or Receive Side Coalescing (RSC)
 58-------------------------------------------------------------
 59  The driver supports LRO/RSC in the vSwitch feature. It reduces the per packet
 60  processing overhead by coalescing multiple TCP segments when possible. The
 61  feature is enabled by default on VMs running on Windows Server 2019 and
 62  later. It may be changed by ethtool command::
 63
 64	ethtool -K eth0 lro on
 65	ethtool -K eth0 lro off
 66
 67SR-IOV support
 68--------------
 69  Hyper-V supports SR-IOV as a hardware acceleration option. If SR-IOV
 70  is enabled in both the vSwitch and the guest configuration, then the
 71  Virtual Function (VF) device is passed to the guest as a PCI
 72  device. In this case, both a synthetic (netvsc) and VF device are
 73  visible in the guest OS and both NIC's have the same MAC address.
 74
 75  The VF is enslaved by netvsc device.  The netvsc driver will transparently
 76  switch the data path to the VF when it is available and up.
 77  Network state (addresses, firewall, etc) should be applied only to the
 78  netvsc device; the slave device should not be accessed directly in
 79  most cases.  The exceptions are if some special queue discipline or
 80  flow direction is desired, these should be applied directly to the
 81  VF slave device.
 82
 83Receive Buffer
 84--------------
 85  Packets are received into a receive area which is created when device
 86  is probed. The receive area is broken into MTU sized chunks and each may
 87  contain one or more packets. The number of receive sections may be changed
 88  via ethtool Rx ring parameters.
 89
 90  There is a similar send buffer which is used to aggregate packets
 91  for sending.  The send area is broken into chunks, typically of 6144
 92  bytes, each of section may contain one or more packets. Small
 93  packets are usually transmitted via copy to the send buffer. However,
 94  if the buffer is temporarily exhausted, or the packet to be transmitted is
 95  an LSO packet, the driver will provide the host with pointers to the data
 96  from the SKB. This attempts to achieve a balance between the overhead of
 97  data copy and the impact of remapping VM memory to be accessible by the
 98  host.
 99
100XDP support
101-----------
102  XDP (eXpress Data Path) is a feature that runs eBPF bytecode at the early
103  stage when packets arrive at a NIC card. The goal is to increase performance
104  for packet processing, reducing the overhead of SKB allocation and other
105  upper network layers.
106
107  hv_netvsc supports XDP in native mode, and transparently sets the XDP
108  program on the associated VF NIC as well.
109
110  Setting / unsetting XDP program on synthetic NIC (netvsc) propagates to
111  VF NIC automatically. Setting / unsetting XDP program on VF NIC directly
112  is not recommended, also not propagated to synthetic NIC, and may be
113  overwritten by setting of synthetic NIC.
114
115  XDP program cannot run with LRO (RSC) enabled, so you need to disable LRO
116  before running XDP::
117
118	ethtool -K eth0 lro off
119
120  XDP_REDIRECT action is not yet supported.