> -----Original Message----- > From: Ilias Apalodimas <ilias.apalodi...@linaro.org> > Sent: Monday, January 7, 2019 12:45 PM > To: Ioana Ciocoi Radulescu <ruxandra.radule...@nxp.com> > Cc: Jesper Dangaard Brouer <bro...@redhat.com>; > netdev@vger.kernel.org; da...@davemloft.net; Ioana Ciornei > <ioana.cior...@nxp.com>; dsah...@gmail.com; Camelia Alexandra Groza > <camelia.gr...@nxp.com> > Subject: Re: Explaining the XDP page-requirement (Was: [PATCH v2 net-next > 0/8] dpaa2-eth: Introduce XDP support) > > Hi Ioana, > > > > 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. > > If you have time, we can discuss merging whatever hardware features are > not > supported in the page_pool API and use that to allocate pages?
Sure. I'd like to first transition to plain page allocations instead of napi_alloc_frag() and validate that's ok (I've been meaning to try that for a while now but haven't got around to it yet), and then we can explore how that might be integrated in page_pool. Thanks, Ioana
