On Fri, 8 May 2020 10:05:46 +0800 Jason Wang <jasowang@xxxxxxxxxx> wrote: > On 2020/5/7 上午4:34, Michael S. Tsirkin wrote: > > On Thu, Apr 30, 2020 at 01:22:08PM +0200, Jesper Dangaard Brouer wrote: > >> The virtio_net driver is running inside the guest-OS. There are two > >> XDP receive code-paths in virtio_net, namely receive_small() and > >> receive_mergeable(). The receive_big() function does not support XDP. > >> > >> In receive_small() the frame size is available in buflen. The buffer > >> backing these frames are allocated in add_recvbuf_small() with same > >> size, except for the headroom, but tailroom have reserved room for > >> skb_shared_info. The headroom is encoded in ctx pointer as a value. > >> > >> In receive_mergeable() the frame size is more dynamic. There are two > >> basic cases: (1) buffer size is based on a exponentially weighted > >> moving average (see DECLARE_EWMA) of packet length. Or (2) in case > >> virtnet_get_headroom() have any headroom then buffer size is > >> PAGE_SIZE. The ctx pointer is this time used for encoding two values; > >> the buffer len "truesize" and headroom. In case (1) if the rx buffer > >> size is underestimated, the packet will have been split over more > >> buffers (num_buf info in virtio_net_hdr_mrg_rxbuf placed in top of > >> buffer area). If that happens the XDP path does a xdp_linearize_page > >> operation. > >> > >> Cc: Jason Wang<jasowang@xxxxxxxxxx> > >> Signed-off-by: Jesper Dangaard Brouer<brouer@xxxxxxxxxx> > > Acked-by: Michael S. Tsirkin<mst@xxxxxxxxxx> > > > Note that we do: > > xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len; > > So using PAGE_SIZE here is probably not correct. Yes, you are correct. I will fix this up in V3. We need to adjust/reduce xdp.frame_sz with these offsets, as frame_sz is an offset size from xdp.data_hard_start. Thanks for pointing this out again, I will fix. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
