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

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Date:   Wed, 9 Jan 2019 16:25:11 +0200
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Madalin-cristian Bucur <madalin.bucur@....com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Ioana Ciocoi Radulescu <ruxandra.radulescu@....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)

Hi Madalin,

> > > 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)?

Yes, both Intel on their ixgbe and i40e driver and mellanox on mlx5 support
this.

Cheers
/Ilias