Subject: Re: [RFC PATCH 00/14] Introducing AF_PACKET V4 support (AF_XDP or AF_CHANNEL?)

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 14 Nov 2017 18:19:22 +0100

On Mon, 13 Nov 2017 22:07:47 +0900 Björn Töpel <[email-masked]> wrote:

> I'll summarize the major points, that we'll address in the next RFC
> below.
> 
> * Instead of extending AF_PACKET with yet another version, introduce a
>   new address/packet family. As for naming had some name suggestions:
>   AF_CAPTURE, AF_CHANNEL, AF_XDP and AF_ZEROCOPY. We'll go for
>   AF_ZEROCOPY, unless there're no strong opinions against it.

I mostly like AF_CHANNEL and AF_XDP. I do know XDP is/have-evolved-into
a kernel-side facility, that moves XDP-frames/packets _inside_ the
kernel.

*BUT* I've always imagined, that we would create a "channel" to
userspace.  By using XDP_REDIRECT to choose what frames get redirected
into which userspace "channel" (new channel-map type).  Userspace
pre-allocate and register memory/pages exactly like this patchset.

[Step-1]: (non-ZC) XDP_REDIRECT need to copy frame-data into userspace
memory pages.  And update your packet_array etc. (Use map-flush to get
RX bulking).

[Step 2]: (ZC) Userspace call driver NDO to register pages. The
XDP_REDIRECT action happens in driver, and can have knowledge about
RX-ring.  It can know if this RX-ring is Zero-Copy enabled and can skip
the copy-step.

> * No explicit zerocopy enablement. Use the zeropcopy path if
>   supported, if not -- fallback to the skb path, for netdevs that
>   don't support the required ndos.

When driver does not support NDO in above model. I think, that there
will still be a significant performance boost for the non-ZC variant.
Even-though we need a copy-operation, because there are no memory
allocations.  As userspace have preallocated and registered pages with
the kernel (and mem-limits are implicit via mem-size reg by userspace).

> * Do not introduce a new XDP action XDP_PASS_TO_KERNEL, instead use
>   XDP redirect map call with ingress flag.

In above model, XDP_REDIRECT is used for filtering into a userspace
"channel".  If ZC gets enabled on a RX-ring queue, then XDP_PASS have
to do a copy (RX-ring knowledge is avail), like you describe with
XDP_PASS_TO_KERNEL.

> * Extend the XDP redirect to support explicit allocator/destructor
>   functions. Right now, XDP redirect assumes that the page allocator
>   was used, and the XDP redirect cleanup path is decreasing the page
>   count of the XDP buffer. This assumption breaks for the zerocopy
>   case.

Yes, please.  If XDP_REDIRECT get call a destructor call-back, then we
can allow XDP_REDIRECT out another net_device, even-when ZC is enabled
on a RX-ring queue.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer