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Re: [Xen-devel] ip/udp checksum offload from minios guest

On Tue, 2011-03-29 at 23:26 +0100, Anil Madhavapeddy wrote:
> Thanks, this is all clear now. I was not setting NETTXF_data_validated
> along with NETTXF_csum_blank in my transmit path, which was confusing
> the backend.

Right, that would have ended up confusing things, I think. c_s && !d_v
still ends up a CHECKSUM_PARTIAL in relatively recent backends. I'm not
sure that has always been implemented correctly though.

In recent netback there is fixup for the case where a GSO packet sets
csum_blank but not data_validated which recalculates the partial
checksum (since Linux requires that all GSO SKBs are CHECKSUM_PARTIAL to
simplify the software GSO checksum stuff). That's only in the GSO case
though since we can't detect it otherwise.

> Ian: yes the IPv4 checksum is of course only over the header. I wrote
> the code right, and then re-read it wrong while hacking on a flight :)

Easily done!

> The ICMP errors confused me (since I was setting NETTXF_csum_blank for
> those too and the checksum functions error out).

Yeah, don't do that ;-)


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