Re: [PATCH 00/11] XDP unaligned chunk placement support

From: "Björn Töpel" <bjorn.topel@intel.com>
To: "Laatz, Kevin" <kevin.laatz@intel.com>,
	Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Jonathan Lemon <jonathan.lemon@gmail.com>,
	netdev@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
	magnus.karlsson@intel.com, bpf@vger.kernel.org,
	intel-wired-lan@lists.osuosl.org, bruce.richardson@intel.com,
	ciara.loftus@intel.com
Subject: Re: [PATCH 00/11] XDP unaligned chunk placement support
Date: Fri, 28 Jun 2019 18:51:37 +0200	[thread overview]
Message-ID: <f6fb0870-b5b4-9aba-bfb5-b4248a95da79@intel.com> (raw)
In-Reply-To: <f0ca817a-02b4-df22-d01b-7bc07171a4dc@intel.com>

On 2019-06-28 18:19, Laatz, Kevin wrote:
> On 27/06/2019 22:25, Jakub Kicinski wrote:
>> On Thu, 27 Jun 2019 12:14:50 +0100, Laatz, Kevin wrote:
>>> On the application side (xdpsock), we don't have to worry about the user
>>> defined headroom, since it is 0, so we only need to account for the
>>> XDP_PACKET_HEADROOM when computing the original address (in the default
>>> scenario).
>> That assumes specific layout for the data inside the buffer.  Some NICs
>> will prepend information like timestamp to the packet, meaning the
>> packet would start at offset XDP_PACKET_HEADROOM + metadata len..
> 
> Yes, if NICs prepend extra data to the packet that would be a problem for
> using this feature in isolation. However, if we also add in support for 
> in-order
> RX and TX rings, that would no longer be an issue. However, even for NICs
> which do prepend data, this patchset should not break anything that is 
> currently
> working.

(Late on the ball. I'm in vacation mode.)

In your example Jakub, how would this look in XDP? Wouldn't the
timestamp be part of the metadata (xdp_md.data_meta)? Isn't
data-data_meta (if valid) <= XDP_PACKET_HEADROOM? That was my assumption.

There were some discussion on having meta data length in the struct
xdp_desc, before AF_XDP was merged, but the conclusion was that this was
*not* needed, because AF_XDP and the XDP program had an implicit
contract. If you're running AF_XDP, you also have an XDP program running
and you can determine the meta data length (and also getting back the
original buffer).

So, today in AF_XDP if XDP metadata is added, the userland application
can look it up before the xdp_desc.addr (just like regular XDP), and how
the XDP/AF_XDP application determines length/layout of the metadata i
out-of-band/not specified.

This is a bit messy/handwavy TBH, so maybe adding the length to the
descriptor *is* a good idea (extending the options part of the
xdp_desc)? Less clean though. OTOH the layout of the meta data still
need to be determined.

Björn