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Date: Fri, 28 Jun 2019 18:51:37 +0200 From: Björn Töpel <bjorn.topel@...el.com> To: "Laatz, Kevin" <kevin.laatz@...el.com>, Jakub Kicinski <jakub.kicinski@...ronome.com> Cc: Jonathan Lemon <jonathan.lemon@...il.com>, netdev@...r.kernel.org, ast@...nel.org, daniel@...earbox.net, magnus.karlsson@...el.com, bpf@...r.kernel.org, intel-wired-lan@...ts.osuosl.org, bruce.richardson@...el.com, ciara.loftus@...el.com Subject: Re: [PATCH 00/11] XDP unaligned chunk placement support 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
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