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
