Re: Questions on XDP

On 17-02-16 12:41 PM, Alexander Duyck wrote:

So I'm in the process of working on enabling XDP for the Intel NICs
and I had a few questions so I just thought I would put them out here
to try and get everything sorted before I paint myself into a corner.

Added Daniel.

So my first question is why does the documentation mention 1 frame per
page for XDP?  Is this with the intention at some point to try and
support page flipping into user space, or is it supposed to have been
for the use with an API such as the AF_PACKET mmap stuff?  If I am not
mistaken the page flipping has been tried in the past and failed, and
as far as the AF_PACKET stuff my understanding is that the pages had
to be mapped beforehand so it doesn't gain us anything without a
hardware offload to a pre-mapped queue.

+1 here. The implementation for virtio does not use page per packet and
works fine. And agreed AF_PACKET does not require it.

If anyone has page-flipping code I would be happy to benchmark it.

Second I was wondering about supporting jumbo frames and scatter
gather.  Specifically if I let XDP handle the first 2-3K of a frame,
and then processed the remaining portion of the frame following the
directive set forth based on the first frame would that be good enough
to satisfy XDP or do I actually have to support 1 linear buffer
always.

For now yes. But, I need a solution to support 64k TSO packets or else
VM to VM traffic is severely degraded in my vswitch use case.

Finally I was looking at xdp_adjust_head.  From what I can tell all
that is technically required to support it is allowing the head to be
adjusted either in or out.  I'm assuming there is some amount of
padding that is preferred.  With the setup I have currently I am
guaranteeing at least NET_SKB_PAD + NET_IP_ALIGN, however I have found
that there should be enough room for 192 bytes on an x86 system if I
am using a 2K buffer.  I'm just wondering if that is enough padding or
if we need more for XDP.

Not surprisingly I'm also in agreement here it would help the ixgbe
implementation out.

Anyway sorry for the stupid questions but I haven't been paying close
of attention to this and was mostly focused on the DMA bits needed to
support this so now I am playing catch-up.

None of the above are stupid IMO. Let me send out the ixgbe implementation
later this afternoon so you can have a look at my interpretation of the
rules.

- Alex