Date:	Sun, 28 Sep 2014 21:11:01 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Or Gerlitz <or.gerlitz@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 5/7] net: Add GSO support for UDP tunnels with checksum

On Sat, Sep 27, 2014 at 1:39 PM, Or Gerlitz <or.gerlitz@...il.com> wrote:
> On Sat, Sep 27, 2014 at 12:57 AM, Tom Herbert <therbert@...gle.com> wrote:
>> On Fri, Sep 26, 2014 at 2:05 PM, Or Gerlitz <or.gerlitz@...il.com> wrote:
>>> On Thu, Jun 5, 2014 at 3:20 AM, Tom Herbert <therbert@...gle.com> wrote:
>
>>>> Added a new netif feature for GSO_UDP_TUNNEL_CSUM. This indicates
>>>> that a device is capable of computing the UDP checksum in the
>>>> encapsulating header of a UDP tunnel.
>
>>> Do we have upstream driver that supports GSO_UDP_TUNNEL_CSUM? did you
>>> had such driver/patch while doing this patches? when a driver
>>> advertizes that bit, should they look over the xmit path on the new
>>> encap_hdr_csum bit?
>
>> No, no, and encap_hdr_csum should only be set with
>> SKB_GSO_UDP_TUNNEL_CSUM or SKB_GSO_GRE_CSUM.
>
> I'm still trying to dig the bigger picture w.r.t checksum of the outer
> UDP packet from the patches -- if I got it right, once these patches
> were picked upstream, there's a scheme where the kernel either
> computes this checksum or let the device do that - when they advertize
> NETIF_F_GSO_YYY_CSUM  and in that case
> (skb->encap_hdr_csum == true) should be interpreted as a directive to
> do that, right?

NETIF_F_GSO_UDP_CSUM does indicate that device is capable of setting
outer UDP checksum. gso_type in an skbuff indicates to driver that the
outer checksum needs to be computed (e.g. SKB_GSO_UDP_TUNNEL_CSUM).
skb->encap_hdr_csum is only used in software GSO, drivers should not
care about this.

>
> So what happens when the device isn't capable to compute that checksum
> (e.g they don't set _GSO_UDP_TUNNEL_CSUM) but they do advertize the
> GSO_UDP_TUNNEL bit?
>
Then for tunnels configured to use outer checksum software GSO would be used.

> I was worried that we can run into this scheme - the stack computes
> the outer checksum for the giant 64K UDP chunck that encapsulate a 64K
> TCP segment, but when the NIC will issue the segmentation, they will
> likely to just repeat ~40 times (64K/1500) the original udp checksum
> for the packets they send, which will be treated as bad checksum on
> the receiving end, bad.
>
This shouldn't happen since the driver would not advertise
SKB_GSO_UDP_TUNNEL_CSUM.

We can add some comments in skbuff.h to clarify some of the GSO types.

Tom

>> find . -name '*.[ch]' -exec fgrep -l GSO_UDP_TUNNEL_CSUM {} \;
>> returns nothing.
>
> grepping I know, I was just hoping you were able to test such a sensitive change
> with HW that cause both code parts (outer udp checksum offloaded vs.
> non-offloaded) to be exercised
>
>> find . -name '*.[ch]' -exec fgrep -l GSO_UDP_TUNNEL {} \;
>> returns
>>
>> ./ethernet/intel/i40e/i40e_main.c
>> ./ethernet/broadcom/bnx2x/bnx2x_main.c
>> ./ethernet/qlogic/qlcnic/qlcnic_main.c
>> ./ethernet/mellanox/mlx4/en_netdev.c
>> ./ethernet/emulex/benet/be_main.c
>
> cool, I see here four 40Gbs NICs that support GSO offloading of VXLAN
> traffic, each of them can serve you for testing new developments you
> do in that area.
>
> Or.
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