On Fri, 4 Sep 2020 10:30:48 -0600
David Ahern [off-list ref] wrote:

On 9/4/20 9:59 AM, Jesper Dangaard Brouer wrote:

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dev_rx for example seems like it could just be the netdev
index rather than a pointer or perhaps can be removed completely. I
believe it is only used for 1 use case (redirects to CPUMAP); maybe that
code can be refactored to handle the dev outside of xdp_frame.  

The dev_rx is needed when creating an SKB from a xdp_frame (basically
skb->dev = rx_dev). Yes, that is done in cpumap, but I want to
generalize this.  The veth also creates SKBs from xdp_frame, but use
itself as skb->dev.

And yes, we could save some space storing the index instead, and trade
space for cycles in a lookup.  

I think this can be managed without adding a reference to the xdp_frame.
I'll start a separate thread on that.

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As for frame_sz, why does it need to be larger than a u16?  

Because PAGE_SIZE can be 64KiB on some archs.
  

I also believe syzbot managed to create packets for generic-XDP with
frame_sz 128KiB, which was a bit weird (it's on my todo list to
investigate and fix).

ok, is there any alignment requirement? can frame_sz be number of 32-bit
words? I believe bit shifts are cheap.

No that is not possible, because some drivers and generic-XDP have a
fully dynamic frame_sz.

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