netdev - How exactly does CHECKSUM

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Date:	Tue, 30 Sep 2014 12:51:49 +0000
From:	Yuval Mintz <Yuval.Mintz@...gic.com>
To:	"Tom Herbert (Partner - google)" <therbert@...gle.com>,
	netdev <netdev@...r.kernel.org>
Subject: How exactly does CHECKSUM_COMPLETE works?

Hi,

I was pondering what it would take for a network device support
CHECKSUM_COMPLETE on its Rx path. And I simply failed to understand the
specifics when using the napi_gro_receive() interface.

Scenario 1 - TCP/IPv4 packet. This will eventually reach tcp4_gro_receive(), and from
there into skb_gro_checksum_validate().
Since this SKB is marked as COMPLETE, the pseudo TCP checksum will be computed
and skb_gro_checksum_validate_complete() be called.
Now, it seems to me as if the only way by which the flow won't actually try to
re-calculate the checksum over TCP header and payload is if NAPI_GRO_CB(skb)->csum
is actually the one's complement to the TCP pseudo header. But that sounds.... odd,
since that's the csum that should have been listed on the SKB - and it doesn't protect
any of the data, only the "metadata".

Scenario 2 - IP-GRE packet, with CSUM option on the GRE header.
This scenario will also eventually reach skb_gro_checksum_validate_complete(),
this time via gre_gro_receive().
This one is even odder, since in order for the flow not to re-calculate the checksum
The driver should have set the SKB's csum to 0xffffffff [as it's using the
null_compute_pseudo() which will return a pseudo_csum of 0].

>From the comment in skbuff.h regarding CHECKSUM_COMPLETE it looks as if all
the HW needs to do is calculate the checksum over the entire data, starting at the
network header.

So, to summarize my questions -
1. What should a driver set as the SKBs csum value when passing CHECKSUM_COMPLETE?
2. Does the GRO flow for non-encapsulated packets work for packets marked as
CHECKSUM_COMPLETE? And by work, I obviously mean 'doesn't re-calculate checksums'.
3. Same as question 2, but for encapsulated packets.

Thanks,
Yuval

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