From: Willem de Bruijn

Sent: 23 September 2023 07:59

On Fri, Sep 22, 2023 at 2:01 PM David Howells [off-list ref] wrote:

quoted

David Laight [off-list ref] wrote:

quoted

quoted

 (8) Move the copy-and-csum code to net/ where it can be in proximity with
     the code that uses it.  This eliminates the code if CONFIG_NET=n and
     allows for the slim possibility of it being inlined.

 (9) Fold memcpy_and_csum() in to its two users.

(10) Move csum_and_copy_from_iter_full() out of line and merge in
     csum_and_copy_from_iter() since the former is the only caller of the
     latter.

I thought that the real idea behind these was to do the checksum
at the same time as the copy to avoid loading the data into the L1
data-cache twice - especially for long buffers.
I wonder how often there are multiple iov[] that actually make
it better than just check summing the linear buffer?

It also reduces the overhead for finding the data to checksum in the case the
packet gets split since we're doing the checksumming as we copy - but with a
linear buffer, that's negligible.

quoted

I had a feeling that check summing of udp data was done during
copy_to/from_user, but the code can't be the copy-and-csum here
for that because it is missing support form odd-length buffers.

Is there a bug there?

No, I misread the code - i shouldn't scan patches when I'd
got a viral head code...

...

quoted

You may be right.  That's more a question for the networking folks than for
me.  It's entirely possible that the checksumming code is just not used on
modern systems these days.

Maybe Willem can comment since he's the UDP maintainer?

Perhaps these days it is more relevant to embedded systems than high
end servers.

The checksum and copy are done together.
I probably missed it because the function isn't passed the
old checksum (which it can pretty much process for free).
Instead the caller is adding it afterwards - which involves
and extra explicit csum_add().

The x86-x84 ip checksum loops are all horrid though.
The unrolling in them is so 1990's.
With the out-of-order pipeline the memory accesses tend
to take care of themselves.
Not to mention that a whole raft of (now oldish) cpu take two
clocks to execute 'adc'.

	David

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