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

On Thu, Nov 25, 2021 at 1:41 AM David Laight [off-list ref] wrote:

From: Eric Dumazet

quoted

Sent: 24 November 2021 20:25

From: Eric Dumazet <redacted>

Remove one pair of add/adc instructions and their dependency
against carry flag.

We can leverage third argument to csum_partial():

  X = csum_block_sub(X, csum_partial(start, len, 0), 0);

  -->

  X = csum_block_add(X, ~csum_partial(start, len, 0), 0);

  -->

  X = ~csum_partial(start, len, ~X);

That doesn't seem to refer to the change in this file.

It is describing the change.

The first step, of copying first the __skb_postpull_rcsum(skb, start,
len, 0) content
into __skb_postpull_rcsum() was kind of obvious.

        David

quoted

Signed-off-by: Eric Dumazet <redacted>
---
 include/linux/skbuff.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index eba256af64a577b458998845f2dc01a5ec80745a..eae4bd3237a41cc1b60b44c91fbfe21dfdd8f117 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -3485,7 +3485,11 @@ __skb_postpull_rcsum(struct sk_buff *skb, const void *start, unsigned int len,
 static inline void skb_postpull_rcsum(struct sk_buff *skb,
                                    const void *start, unsigned int len)
 {
-     __skb_postpull_rcsum(skb, start, len, 0);
+     if (skb->ip_summed == CHECKSUM_COMPLETE)
+             skb->csum = ~csum_partial(start, len, ~skb->csum);
+     else if (skb->ip_summed == CHECKSUM_PARTIAL &&
+              skb_checksum_start_offset(skb) < 0)
+             skb->ip_summed = CHECKSUM_NONE;
 }

 static __always_inline void
--

2.34.0.rc2.393.gf8c9666880-goog

-
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