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

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Message-ID: <AM0PR04MB49946E77A8D8D3AF083C8E7594890@AM0PR04MB4994.eurprd04.prod.outlook.com>
Date:   Mon, 7 Jan 2019 10:34:31 +0000
From:   Ioana Ciocoi Radulescu <ruxandra.radulescu@....com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
CC:     Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Ioana Ciornei <ioana.ciornei@....com>,
        "dsahern@...il.com" <dsahern@...il.com>,
        Camelia Alexandra Groza <camelia.groza@....com>
Subject: RE: Explaining the XDP page-requirement (Was: [PATCH v2 net-next 0/8]
 dpaa2-eth: Introduce XDP support)

> -----Original Message-----
> From: Jesper Dangaard Brouer <brouer@...hat.com>
> Sent: Friday, December 21, 2018 5:31 PM
> To: Ioana Ciocoi Radulescu <ruxandra.radulescu@....com>
> Cc: Ilias Apalodimas <ilias.apalodimas@...aro.org>; netdev@...r.kernel.org;
> davem@...emloft.net; Ioana Ciornei <ioana.ciornei@....com>;
> dsahern@...il.com; Camelia Alexandra Groza <camelia.groza@....com>;
> brouer@...hat.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.radulescu@....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