• Vitaly Kuznetsov's avatar
    xen-netfront: avoid packet loss when ethernet header crosses page boundary · fd07160b
    Vitaly Kuznetsov authored
    Small packet loss is reported on complex multi host network configurations
    including tunnels, NAT, ... My investigation led me to the following check
    in netback which drops packets:
    
            if (unlikely(txreq.size < ETH_HLEN)) {
                    netdev_err(queue->vif->dev,
                               "Bad packet size: %d\n", txreq.size);
                    xenvif_tx_err(queue, &txreq, extra_count, idx);
                    break;
            }
    
    But this check itself is legitimate. SKBs consist of a linear part (which
    has to have the ethernet header) and (optionally) a number of frags.
    Netfront transmits the head of the linear part up to the page boundary
    as the first request and all the rest becomes frags so when we're
    reconstructing the SKB in netback we can't distinguish between original
    frags and the 'tail' of the linear part. The first SKB needs to be at
    least ETH_HLEN size. So in case we have an SKB with its linear part
    starting too close to the page boundary the packet is lost.
    
    I see two ways to fix the issue:
    - Change the 'wire' protocol between netfront and netback to start keeping
      the original SKB structure. We'll have to add a flag indicating the fact
      that the particular request is a part of the original linear part and not
      a frag. We'll need to know the length of the linear part to pre-allocate
      memory.
    - Avoid transmitting SKBs with linear parts starting too close to the page
      boundary. That seems preferable short-term and shouldn't bring
      significant performance degradation as such packets are rare. That's what
      this patch is trying to achieve with skb_copy().
    Signed-off-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
    Acked-by: default avatarDavid Vrabel <david.vrabel@citrix.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    fd07160b
xen-netfront.c 54.2 KB