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"). -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
