be50da3e9d
Virtio spec introduced a feature VIRTIO_NET_F_GUEST_HDRLEN which when set implicates that device benefits from knowing the exact size of the header. For compatibility, to signal to the device that the header is reliable driver also needs to set this feature. Without this feature set by driver, device has to figure out the header size itself. Quoting the original virtio spec: "hdr_len is a hint to the device as to how much of the header needs to be kept to copy into each packet" "a hint" might not be clear for the reader what does it mean, if it is "maybe like that" of "exactly like that". This feature just makes it crystal clear and let the device count on the hdr_len being filled up by the exact length of header. Also note the spec already has following note about hdr_len: "Due to various bugs in implementations, this field is not useful as a guarantee of the transport header size." Without this feature the device needs to parse the header in core data path handling. Accurate information helps the device to eliminate such header parsing and directly use the hardware accelerators for GSO operation. virtio_net_hdr_from_skb() fills up hdr_len to skb_headlen(skb). The driver already complies to fill the correct value. Introduce the feature and advertise it. Note that virtio spec also includes following note for device implementation: "Caution should be taken by the implementation so as to prevent a malicious driver from attacking the device by setting an incorrect hdr_len." There is a plan to support this feature in our emulated device. A device of SolidRun offers this feature bit. They claim this feature will save the device a few cycles for every GSO packet. Link: https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.html#x1-230006x3 Signed-off-by: Jiri Pirko <jiri@nvidia.com> Reviewed-by: Parav Pandit <parav@nvidia.com> Reviewed-by: Alvaro Karsz <alvaro.karsz@solid-run.com> Acked-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Willem de Bruijn <willemb@google.com> Link: https://lore.kernel.org/r/20230309094559.917857-1-jiri@resnulli.us Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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