On Mon, 2021-06-21 at 11:52 +0100, David Woodhouse wrote:

Firstly, I don't think I can set IFF_VNET_HDR on the tun device after
opening it. So my model of "open the tun device, then *see* if we can
use vhost to accelerate it" doesn't work.

I tried setting VHOST_NET_F_VIRTIO_NET_HDR in the vhost features
instead, but that gives me a weird failure mode where it drops around
half the incoming packets, and I haven't yet worked out why.

FWIW that problem also goes away if I set TUNSNDBUF and avoid the XDP
data path.