* IP GRO verifies csum again?
@ 2019-07-18 7:49 Jacob Wen
2019-07-18 8:27 ` Eric Dumazet
0 siblings, 1 reply; 2+ messages in thread
From: Jacob Wen @ 2019-07-18 7:49 UTC (permalink / raw)
To: netdev; +Cc: herbert
Hi,
inet_gro_receive verifies IP csum but a NIC already did so and set
CHECKSUM_UNNECESSARY.
https://github.com/torvalds/linux/blob/v5.2/net/ipv4/af_inet.c#L1432-L1433
if (unlikely(ip_fast_csum((u8 *)iph, 5)))
goto out_unlock;
Is this a bug?
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: IP GRO verifies csum again?
2019-07-18 7:49 IP GRO verifies csum again? Jacob Wen
@ 2019-07-18 8:27 ` Eric Dumazet
0 siblings, 0 replies; 2+ messages in thread
From: Eric Dumazet @ 2019-07-18 8:27 UTC (permalink / raw)
To: Jacob Wen, netdev; +Cc: herbert
On 7/18/19 9:49 AM, Jacob Wen wrote:
> Hi,
>
> inet_gro_receive verifies IP csum but a NIC already did so and set CHECKSUM_UNNECESSARY.
>
>
> https://github.com/torvalds/linux/blob/v5.2/net/ipv4/af_inet.c#L1432-L1433
>
> if (unlikely(ip_fast_csum((u8 *)iph, 5)))
>
> goto out_unlock;
>
>
> Is this a bug?
>
This checksum validates the TCP one, which is the real cost, since we need to touch all
the packet.
We do not bother 'offloading' IPV4 checksum over 20 bytes or so, since in modern cpus,
having the cache line hot in cpu caches means the checksum is almost free.
Adding a test here would not always be a win, say for CHECKSUM_COMPLETE cases,
which we try to generalize in favor of old CHECKSUM_UNNECESSARY.
^ permalink raw reply [flat|nested] 2+ messages in thread
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