From: Jesper Dangaard Brouer <brouer@redhat.com>
To: David Miller <davem@davemloft.net>
Cc: mst@redhat.com, shrijeet@gmail.com, tom@herbertland.com,
netdev@vger.kernel.org, shm@cumulusnetworks.com,
roopa@cumulusnetworks.com, nikolay@cumulusnetworks.com,
brouer@redhat.com
Subject: Re: [PATCH net-next RFC WIP] Patch for XDP support for virtio_net
Date: Thu, 27 Oct 2016 10:55:14 +0200 [thread overview]
Message-ID: <20161027105514.4c0374dd@redhat.com> (raw)
In-Reply-To: <20161026.131122.318218609871320141.davem@davemloft.net>
On Wed, 26 Oct 2016 13:11:22 -0400 (EDT)
David Miller <davem@davemloft.net> wrote:
> From: "Michael S. Tsirkin" <mst@redhat.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@redhat.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@gmail.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
next prev parent reply other threads:[~2016-10-27 9:14 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-22 4:07 [PATCH net-next RFC WIP] Patch for XDP support for virtio_net Shrijeet Mukherjee
2016-10-23 16:38 ` Stephen Hemminger
2016-10-24 1:51 ` Shrijeet Mukherjee
2016-10-25 1:10 ` Alexei Starovoitov
2016-10-25 17:36 ` Jakub Kicinski
2016-10-26 13:52 ` Jesper Dangaard Brouer
2016-10-26 16:36 ` Michael S. Tsirkin
2016-10-26 16:52 ` David Miller
2016-10-26 17:07 ` Michael S. Tsirkin
2016-10-26 17:11 ` David Miller
2016-10-27 8:55 ` Jesper Dangaard Brouer [this message]
2016-10-27 21:09 ` John Fastabend
2016-10-27 21:30 ` Michael S. Tsirkin
2016-10-27 21:42 ` David Miller
2016-10-27 22:25 ` Michael S. Tsirkin
2016-10-28 1:35 ` David Miller
2016-10-28 1:43 ` Alexander Duyck
2016-10-28 2:10 ` David Miller
2016-10-28 15:56 ` John Fastabend
2016-10-28 16:18 ` Jakub Kicinski
2016-10-28 18:22 ` Alexei Starovoitov
2016-10-28 20:35 ` Alexander Duyck
2016-10-28 20:42 ` Jakub Kicinski
2016-10-28 20:36 ` Jakub Kicinski
2016-10-29 3:51 ` Shrijeet Mukherjee
2016-10-29 11:25 ` Thomas Graf
2016-11-02 14:27 ` Jesper Dangaard Brouer
2016-11-03 1:28 ` Shrijeet Mukherjee
2016-11-03 4:11 ` Michael S. Tsirkin
2016-11-03 6:44 ` John Fastabend
2016-11-03 22:20 ` John Fastabend
2016-11-03 22:42 ` Michael S. Tsirkin
2016-11-03 23:29 ` John Fastabend
2016-11-04 0:34 ` Michael S. Tsirkin
2016-11-04 23:05 ` John Fastabend
2016-11-06 6:50 ` Michael S. Tsirkin
2016-10-28 17:11 ` David Miller
2016-10-30 22:53 ` Michael S. Tsirkin
2016-11-02 14:01 ` Jesper Dangaard Brouer
2016-11-02 16:06 ` Alexander Duyck
2016-10-28 0:02 ` Shrijeet Mukherjee
2016-10-28 0:46 ` Shrijeet Mukherjee
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