netdev - Re: [PATCH v2 net-next 0/7] net: foo-over-udp (fou)

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Date:	Tue, 16 Sep 2014 22:14:21 +0300
From:	Or Gerlitz <gerlitz.or@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Jesse Gross <jesse@...ira.com>, David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next 0/7] net: foo-over-udp (fou)

On Tue, Sep 16, 2014 at 9:34 PM, Tom Herbert <therbert@...gle.com> wrote:
> On Tue, Sep 16, 2014 at 5:44 AM, Or Gerlitz <gerlitz.or@...il.com> wrote:
>> On Tue, Sep 16, 2014 at 1:44 AM, Jesse Gross <jesse@...ira.com> wrote:
>>> On Mon, Sep 15, 2014 at 12:15 PM, Tom Herbert <therbert@...gle.com> wrote:
>>
>>>> My interpretation is that NETIF_F_GSO_UDP_TUNNEL means L3/L4
>>>> encapsulation over UDP, not VXLAN.
>>>> If the NIC implements things properly following the generic interface then I believe it should work
>>>> with various flavors of UDP encapsulation (FOU, GUE, VXLAN, VXLAN-gpe,
>>>> geneve, LISP, L2TP, nvgre, or whatever else people might dream up).
>>>> This presumes that any encapsulation headers doesn't require any per
>>>> segment update (so no GRE csum for instance). The stack will set up
>>>> inner headers as needed, which should enough to provide to devices the
>>>> offsets inner IP and TCP header which are needed for the the TSO
>>>> operation (outer IP and UDP can be deduced also).
>>
>>
>>
>>> From the NICs that I am familiar with this is mostly true. The main
>>> part that is missing from the current implementation is a length
>>> limit: just because the hardware can skip over headers to an offset
>>> doesn't mean that it can do so to an arbitrary depth. For example, in
>>> the NICs that are exposing VXLAN as NETIF_F_GSO_UDP_TUNNEL we can
>>> probably assume that this is limited to 8 bytes. With the Intel NICs
>>> that were just announced with Geneve support, this limit has been
>>> increased to 64. If we add a parameter to the driver interface to
>>> expose this then it should be generic across tunnels.
>>
>> I'm not sure to see why the length limit became our primary concern here...

> Like Jesse mentioned above, looks like some NICs may have assumed all
> encapsulation headers are eight bytes (which allows HW to implement
> everything in fixed offsets). But this length is not a universal
> constant: FOU is zero length encapsulation headers, GUE or geneve is
> variable. The driver should really be checking if NIC can handle the
> length and if it can't perform GSO in software-- I don't think we'll
> need to expose this in the features.

I understand that for some NICs there's a claim saying the essence of
the limitation lies in an assumption on fixed length of the
encapsulation headers  -- and BTW for VXLAN it's 50 (= 14 + 20 + 8 +
8) bytes, not eight. So newer NICs  or new brands of existing NICs
should be more flexible.

If I correctly read your comment "The driver should really be checking
if NIC can handle the length and if it can't perform GSO in software"
as saying that a SW GSO call should be made from within the driver
when they can't serve GSO under some encap scheme -- I don't think
this is the correct track, the driver should advertize up what they
can do in HW so the stack does in SW what's not supported.

Another clarification - so FOU doesn't supersedes GUE? what's their
difference...?

Or.
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