On Thu, Oct 27, 2016 at 02:09:08PM -0700, John Fastabend wrote: > On 16-10-27 01:55 AM, Jesper Dangaard Brouer wrote: > > On Wed, 26 Oct 2016 13:11:22 -0400 (EDT) > > David Miller <da...@davemloft.net> wrote: > > > >> From: "Michael S. Tsirkin" <m...@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" <m...@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 <shrij...@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. > > > > Looks to me the correct conditions can be met by getting the correct > feature negotiation to happen, specifically no VIRTIO_NET_F_MRG_RXBUF > and one of the TSO/ECN/GSO feature bits set the big_packets flag as > Jesper notes. > > Srijeet, are you going to give this a try? I'm trying to get the device > side working by the way on the vhost interface. > > Thanks, > John
Something I'd like to understand is how does XDP address the problem that 100Byte packets are consuming 4K of memory now. -- MST
