Subject: Explaining the XDP page-requirement (Was: [PATCH v2 net-next 0/8] dpaa2-eth: Introduce XDP support)

From: Jesper Dangaard Brouer <[email-masked]>
Date: Fri, 21 Dec 2018 16:31:19 +0100

On Fri, 7 Dec 2018 18:07:49 +0000
Ioana Ciocoi Radulescu <[email-masked]> 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