Re: [PATCH net-next 2/2] net: optimize skb_postpull_rcsum()

On Fri, Dec 03, 2021 at 02:57:04PM +0000, David Laight wrote:

From: Vladimir Oltean

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Sent: 02 December 2021 21:40

...

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Try replacing both ~ with -.
So replace:
		skb->csum = ~csum_partial(start, len, ~skb->csum);
with:
		skb->csum = -csum_partial(start, len, -skb->csum);

That should geneate ~0u instead 0 (if I've got my maths right).

Indeed, replacing both one's complement operations with two's complement
seems to produce correct results (consistent with old code) in all cases
that I am testing with (ICMP, TCP, UDP). Thanks!

You need to generate (or persuade Eric to generate) a patch.
I don't have the right source tree.

Any code that does ~csum_partial() is 'dubious' unless followed
by a check for 0.
The two's compliment negate save the conditional - provided the
offset of 1 can be added in earlier.

	David

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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Eric, could you please send a patch with this change?
If you want and if it helps, I can also help you reproduce this locally
using the dsa_loop mockup driver.