netdev - Re: [PATCH net-next RFC WIP] Patch for XDP support for virtio

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Date:   Thu, 27 Oct 2016 10:55:14 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     mst@...hat.com, shrijeet@...il.com, tom@...bertland.com,
        netdev@...r.kernel.org, shm@...ulusnetworks.com,
        roopa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
        brouer@...hat.com
Subject: Re: [PATCH net-next RFC WIP] Patch for XDP support for virtio_net

On Wed, 26 Oct 2016 13:11:22 -0400 (EDT)
David Miller <davem@...emloft.net> wrote:

> From: "Michael S. Tsirkin" <mst@...hat.com>
> Date: Wed, 26 Oct 2016 20:07:19 +0300
> 
> > On Wed, Oct 26, 2016 at 12:52:45PM -0400, David Miller wrote:  
> >> From: "Michael S. Tsirkin" <mst@...hat.com>
> >> Date: Wed, 26 Oct 2016 19:36:45 +0300
> >>   
> >> > On Wed, Oct 26, 2016 at 03:52:02PM +0200, Jesper Dangaard Brouer wrote:  
> >> >> On Sat, 22 Oct 2016 04:07:23 +0000
> >> >> Shrijeet Mukherjee <shrijeet@...il.com> wrote:
> >> >>   
> >> >> > This patch adds support for xdp ndo and also inserts the xdp program
> >> >> > call into the merged RX buffers and big buffers paths  
> >> >> 
> >> >> I really appreciate you are doing this for virtio_net.
> >> >> 
> >> >> My first question is: Is the (packet) page data writable?
> >> >> (MST might be able to answer?)
> >> >> 
> >> >> As this is currently an XDP requirement[1].    
> >> > 
> >> > I'm not sure I understand what does writable mean.
> >> > Could you explain a bit more pls?
> >> > We do copy data into skb ATM but I plan to change that.  
> >> 
> >> The packet data area must be writable,  
> > 
> > This is the part I don't fully understand.
> > It's in RAM so of course it's writeable.  
> 
> Pages in SKB frag lists are not usually writable, because they share
> space with data from other packets the way drivers usually carve up
> pages to receive packets into.
> 
> It is therefore illegal for the networking code to write into SKB frag
> pages.
> 
> Pages used for XDP processed packets must not have this restriction.
> 
> > We share pages between arbitrary multiple packets. I think that's
> > OK  
> 
> That's exactly what is not allowed with XDP.
> 
> Each packet must be the sole user of a page, otherwise the semantics
> required by XDP are not met.

Looking at the virtio_net.c code, the function call receive_big() might
actually be okay for XDP, unless the incoming packet is larger than
PAGE_SIZE and thus uses several pages (via a linked list in page->private).

The receive_mergeable() does not look compatible with XDP.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer