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. -- 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
