1 .. SPDX-License-Identifier: GPL-2.0
10 This driver is compatible with Windows Server 2012 R2, 2016 and
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.
25 Hyper-V supports receive side scaling. For TCP & UDP, packets can
26 be distributed among available queues based on IP address and port
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.
34 On Azure, fragmented UDP packets have high loss rate with L4
35 hashing. Using L3 hashing is recommended in this case.
37 For example, for UDP over IPv4 on eth0:
39 To include UDP port numbers in hashing::
41 ethtool -N eth0 rx-flow-hash udp4 sdfn
43 To exclude UDP port numbers in hashing::
45 ethtool -N eth0 rx-flow-hash udp4 sd
47 To show UDP hash level::
49 ethtool -n eth0 rx-flow-hash udp4
51 Generic 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
57 Large 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::
64 ethtool -K eth0 lro on
65 ethtool -K eth0 lro off
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.
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
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.
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
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.
107 hv_netvsc supports XDP in native mode, and transparently sets the XDP
108 program on the associated VF NIC as well.
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.
115 XDP program cannot run with LRO (RSC) enabled, so you need to disable LRO
118 ethtool -K eth0 lro off
120 XDP_REDIRECT action is not yet supported.