From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Mon, 19 Oct 2009 22:42:02 +0200

quoted text
> One bit could be corrupted in iph, and ntohl(iph->daddr) becomes 0x7fxxyyzz,
> we then accept a bogus frame. This is a RFC violation.
> 
> This also slows down non loopback devices, adding an extra test.
> 
> ip_fast_csum() is really fast (about 16 instructions).

Also loopback doesn't mean anything.  That packet could
be mirrored and sent externally via packet scheduler
rules and actions.

And for this specific case, the savings are absolutely zero.

We've brought the whole damn IP header into the CPU cache and
that is the real cost.  The calculation is something like 12
instructions, maybe 6 cycles on a modern cpu, which is nothing.
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