Virtio-net driver currently negotiates network offloads on startup via features mechanism and have no ability to disable and re-enable offloads later.
This patch introduced a new control command that allows to configure device network offloads state dynamically. The patch also introduces a new feature flag VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.

VIRTIO_NET_F_CTRL_GUEST_OFFLOADS (2) Control channel offloads reconfiguration support.

control virtqueue “Offloads State Configuration”.

“offloads”是名词，按照名词去阅读spec。

“negotiate”指的是在driver_feature中正确配置好了feature bits。

VIRTIO_NET_F_CTRL_GUEST_OFFLOADS支持动态地更改offloads的状态(enable/disbale)配置。

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// feature bits
#define VIRTIO_NET_F_GUEST_CSUM       1
#define VIRTIO_NET_F_GUEST_TSO4       7
#define VIRTIO_NET_F_GUEST_TSO6       8
#define VIRTIO_NET_F_GUEST_ECN        9
#define VIRTIO_NET_F_GUEST_UFO        10
#define VIRTIO_NET_F_GUEST_USO4       54
#define VIRTIO_NET_F_GUEST_USO6       55
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/*
 * Control network offloads
 *
 * Reconfigures the network offloads that Guest can handle.
 *
 * Available with the VIRTIO_NET_F_CTRL_GUEST_OFFLOADS feature bit.
 *
 * Command data format matches the feature bit mask exactly.
 *
 * See VIRTIO_NET_F_GUEST_* for the list of offloads
 * that can be enabled/disabled.
 */
#define VIRTIO_NET_CTRL_GUEST_OFFLOADS   5
#define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET        0

参考资料:

  1. virtio v1.3
  2. virtio-net: dynamic network offloads configuration