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: <AM0PR04MB555472BCD12F50E30DCBCCDBEC8B0@AM0PR04MB5554.eurprd04.prod.outlook.com>
Date:   Wed, 9 Jan 2019 14:22:56 +0000
From:   Madalin-cristian Bucur <madalin.bucur@....com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Ioana Ciocoi Radulescu <ruxandra.radulescu@....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: netdev-owner@...r.kernel.org <netdev-owner@...r.kernel.org> On
> Behalf Of Jesper Dangaard Brouer
> 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)

Hi Jesper,

is the split page memory model supported now (with two frames per page)?

Thanks,
Madalin

> 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:
> https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.link
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