Re: [PATCH net v4 2/2] selftests/bpf: cover AF_XDP multi-buffer TX software checksum

[Rongzheng Wang <testwrzwrz@gmail.com>]

> Any reason you're not reading the real value from
> /proc/sys/net/core/max_skb_frags ?

The test wants to exercise the maximum supported number of frames, but also
needs to run on every build. CONFIG_MAX_SKB_FRAGS ranges from 17 to 45, so
using the minimum (17, i.e. 18 frames) gives the largest bound that works
everywhere. af_xdp.rst documents the same limit:

  "For copy mode, the maximum supported number of frames in a packet is
   equal to CONFIG_MAX_SKB_FRAGS + 1."

xsk checks MAX_SKB_FRAGS directly (net/xdp/xsk.c), not
net.core.max_skb_frags. I also set net.core.max_skb_frags=4 and ran
xdp_metadata; it still passes, which confirms xsk does not consume that
sysctl. Reading it in this test would not tell us the xsk limit, and would
only reduce coverage if it is lowered.

CONFIG_MAX_SKB_FRAGS is compile-time and has no userspace interface, so 18
frames is the portable upper bound. That is why I used 17.

Thanks,
Rongzheng