Re: [PATCH net-next v2 21/33] virtio_net: add XDP frame size in two code paths

[Jason Wang <jasowang@redhat.com>]

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@redhat.com>
>> Signed-off-by: Jesper Dangaard Brouer<brouer@redhat.com>
> Acked-by: Michael S. Tsirkin<mst@redhat.com>


Note that we do:

         xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;

So using PAGE_SIZE here is probably not correct.

Thanks

>