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? Regards /Ilias
