> -----Original Message----- > From: Jesper Dangaard Brouer <bro...@redhat.com> > 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) > > 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"). >
Thanks a lot, that's great info, especially for someone who hasn't followed so closely xdp development from its beginning. I'll look into updating the dpaa2-eth driver to use one page per frame and see how that goes. Thanks, Ioana
