Re: [Xen-devel] [PATCH] xen-netfront: transmit fully GSO-sized packets

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

On Thu, 2015-03-26 at 11:13 +0000, Jonathan Davies wrote:
> xen-netfront limits transmitted skbs to be at most 44 segments in size. 
> However,
> GSO permits up to 65536 bytes, which means a maximum of 45 segments of 1448
> bytes each. This slight reduction in the size of packets means a slight loss 
> in
> efficiency.
> 
> Since c/s 9ecd1a75d, xen-netfront sets gso_max_size to
>     XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER,
> where XEN_NETIF_MAX_TX_SIZE is 65535 bytes.
> 
> The calculation used by tcp_tso_autosize (and also tcp_xmit_size_goal since 
> c/s
> 6c09fa09d) in determining when to split an skb into two is
>     sk->sk_gso_max_size - 1 - MAX_TCP_HEADER.
> 
> So the maximum permitted size of an skb is calculated to be
>     (XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER) - 1 - MAX_TCP_HEADER.
> 
> Intuitively, this looks like the wrong formula -- we don't need two TCP 
> headers.
> Instead, there is no need to deviate from the default gso_max_size of 65536 as
> this already accommodates the size of the header.
> 
> Currently, the largest skb transmitted by netfront is 63712 bytes (44 segments
> of 1448 bytes each), as observed via tcpdump. This patch makes netfront send
> skbs of up to 65160 bytes (45 segments of 1448 bytes each).
> 
> Fixes: 9ecd1a75d977 ("xen-netfront: reduce gso_max_size to account for max 
> TCP header")
> Signed-off-by: Jonathan Davies <jonathan.davies@xxxxxxxxxx>
> ---
>  drivers/net/xen-netfront.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index e9b960f..fb6e978 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -1279,8 +1279,6 @@ static struct net_device *xennet_create_dev(struct 
> xenbus_device *dev)
>       netdev->ethtool_ops = &xennet_ethtool_ops;
>       SET_NETDEV_DEV(netdev, &dev->dev);
>  
> -     netif_set_gso_max_size(netdev, XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER);
> -
>       np->netdev = netdev;
>  
>       netif_carrier_off(netdev);

Hmm, this partially reverts commit
9ecd1a75d977e2e8c48139c7d3efed183f898d94



Why xennet_change_mtu() is not changed by your patch ?




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