> -----Original Message----- > From: netdev-ow...@vger.kernel.org <netdev-ow...@vger.kernel.org> On > Behalf Of Jesper Dangaard Brouer > Sent: Friday, December 21, 2018 5:31 PM > To: Ioana Ciocoi Radulescu <ruxandra.radule...@nxp.com> > Cc: Ilias Apalodimas <ilias.apalodi...@linaro.org>; > netdev@vger.kernel.org; da...@davemloft.net; Ioana Ciornei > <ioana.cior...@nxp.com>; dsah...@gmail.com; Camelia Alexandra Groza > <camelia.gr...@nxp.com>; bro...@redhat.com > Subject: Explaining the XDP page-requirement (Was: [PATCH v2 net-next 0/8] > dpaa2-eth: Introduce XDP support) > > On Fri, 7 Dec 2018 18:07:49 +0000 > Ioana Ciocoi Radulescu <ruxandra.radule...@nxp.com> wrote: > > > Thanks a lot for the info, will look into this. Do you have any > > pointers as to why the full page restriction exists in the first > > place? Sorry if it's a dumb question, but I haven't found details on > > this and I'd really like to understand it. > > Hi Ioana, > > I promised (offlist) that I would get back to you explaining the XDP > page-requirement... > > There are several reasons for XDP to require frames are backed by a > page. It started out with a focus on gaining speed via simplicity. > > The overall requirement is: XDP frame in physical contigious memory > - which is a requirement from BPF Direct-Access, for validating > correcness. > - Implying you cannot split packet data over several pages. > > An important part of the page-requirement is to allow creating SKB's > outside the driver code. This happen today in both cpumap and veth > (when doing XDP_REDIRECT). And we need to control and limit the > variations, to avoid having to handle all kind of SKB schemes. > Specifically we need enough tailroom for the skb-shared-info. > > In the beginning we had the requirement of: 1-page per XDP frame. > - Gave us a simplified memory model > - Allow us to not touch atomic refcnt on page (always 1) > - Fixed 256 bytes headroom > - This gave us a lot of tailroom, expanding tail was trivial. > > Eventually ixgbe+i40e force us to use a split-page model, allowing two > frames per page. > - This started to complicate memory model > - This unfortunately gave issue of unknown tailroom, which killed the > tailroom expand option. > - Changes XDP headroom to be variable (192 or 256 bytes)
Hi Jesper, is the split page memory model supported now (with two frames per page)? Thanks, Madalin > E.g. I really want to allow bpf_xdp_adjust_tail() to *expand* the > frame size, but after allowing the split-page model, we couldn't allow > this easily. And SKB alloc in cpumap/veth was also complicated by not > knowing (implicit) xdp_frame "hard-end". (We might have to extend > xdp_buff with "data_hard_end"). > > -- > Best regards, > Jesper Dangaard Brouer > MSc.CS, Principal Kernel Engineer at Red Hat > LinkedIn: > https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.link > edin.com%2Fin%2Fbrouer&data=02%7C01%7Cmadalin.bucur%40nxp.com%7C44c593 > 0f8a224fdd063208d66759613b%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C63 > 6810030928918215&sdata=PIdwIEvOAPlyWPScMjOdWiauOp2wAI7QXu9FNJ0SHzs%3D& > amp;reserved=0
