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: <AM0PR04MB499479E078D9E579067A60A794890@AM0PR04MB4994.eurprd04.prod.outlook.com>
Date:   Mon, 7 Jan 2019 10:49:25 +0000
From:   Ioana Ciocoi Radulescu <ruxandra.radulescu@....com>
To:     Ilias Apalodimas <ilias.apalodimas@...aro.org>
CC:     Jesper Dangaard Brouer <brouer@...hat.com>,
        "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: Ilias Apalodimas <ilias.apalodimas@...aro.org>
> Sent: Monday, January 7, 2019 12:45 PM
> To: Ioana Ciocoi Radulescu <ruxandra.radulescu@....com>
> Cc: Jesper Dangaard Brouer <brouer@...hat.com>;
> netdev@...r.kernel.org; davem@...emloft.net; Ioana Ciornei
> <ioana.ciornei@....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)
> 
> 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?

Sure. I'd like to first transition to plain page allocations instead of
napi_alloc_frag() and validate that's ok (I've been meaning to try that for
a while now but haven't got around to it yet), and then we can explore
how that might be integrated in page_pool.

Thanks,
Ioana