Re: [Patch net v3] mlx5: force CHECKSUM_NONE for short ethernet frames

On Mon, Dec 3, 2018 at 11:30 PM Cong Wang [off-list ref] wrote:

On Mon, Dec 3, 2018 at 11:08 PM Eric Dumazet [off-list ref] wrote:

quoted

The hardware has probably validated the L3 & L4 checksum just fine.

Note that if ip_summed is CHECKSUM_UNNECESSARY, the padding bytes (if any)
have no impact on the csum that has been verified by the NIC.

Why? Why does the hardware validates L3/L4 checksum when it
supplies a full-packet checksum? What's its point here?

The point is that the driver author can decide what is best.

For native IP+TCP or IP+UDP, the NIC has the ability to fully
understand the packet and fully validate the checksum.

If it really validates L3/L4 checksum, then a full-packet checksum
is not needed.

Yes, this is exactly what CHECKSUM_UNNECESSARY means.
linux stack does not have to perform the check another time.

For example, no call to csum_partial() is needed, even for IPv6+TCP or IPv6+UDP