netdev - Re: ethtool features: tx-udp_tnl-csum-segmentation and tx-udp

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Date:   Thu, 29 Apr 2021 15:49:38 +0100
From:   Edward Cree <ecree.xilinx@...il.com>
To:     Aya Levin <ayal@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...dia.com>
Cc:     Tariq Toukan <tariqt@...dia.com>, Moshe Shemesh <moshe@...dia.com>,
        Maria Pasechnik <mariap@...dia.com>
Subject: Re: ethtool features: tx-udp_tnl-csum-segmentation and
 tx-udp_tnl-segmentation

On 29/04/2021 10:16, Aya Levin wrote:
> I see a strange behavior when toggling feature flags:
> (1) tx-udp_tnl-csum-segmentation
> (2) tx-udp_tnl-segmentation
...
> What is the role of each feature flag?
IIRC, tx-udp_tnl-segmentation controls whether to do TSO on packets that don't
 have an outer checksum to offload, whereas tx-udp_tnl-csum-segmentation controls
 the same for packets that _do_ need outer checksum offload.  The difference
 being whether gso_type is SKB_GSO_UDP_TUNNEL or SKB_GSO_UDP_TUNNEL_CSUM.

To a first approximation there's one feature flag for each SKB_GSO_* bit, and if
 an skb's gso_type requires a feature that's not enabled on the device, the core
 will segment that skb in software before handing it to the driver.

Documentation/networking/segmentation-offloads.rst may also be useful to read if
 you haven't already.

(And note that the kernel's favourite way for hardware to behave is to instead
 provide GSO_PARTIAL offload / tx-gso-partial, rather than doing protocol-
 ossified offloads for specific kinds of tunnels.)

-ed