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

From: Sergei Shtylyov <[email-masked]>
Date: Thu, 26 Mar 2015 17:14:08 +0300

Hello.

On 3/26/2015 2:13 PM, 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

    c/s == commit?
    Please also specify that commit's summary line in parens.

>      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

    Likewise.

>      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")

    Ah, here's the summary for the first mentioned commit...

> Signed-off-by: Jonathan Davies <[email-masked]>

WBR, Sergei