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

From: Jonathan Davies <[email-masked]>
Date: Thu, 26 Mar 2015 15:08:58 +0000

On 26/03/15 12:05, Eric Dumazet wrote:
> 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 <[email-masked]>
>> ---
>>   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 ?

I think you are right: the mtu calculation suffers from the same 
problem. I believe the value of mtu relates to the size of the whole 
packet, including the header (which is why the value of dev->mtu is 
normally 1500 rather than 1448).

Wei, as the author of commit 9ecd1a75d977 ("xen-netfront: reduce 
gso_max_size to account for max TCP header"), do you agree that the max 
mtu formula should not need to subtract MAX_TCP_HEADER?

Regards,
Jonathan