From: Tom Herbert <t...@herbertland.com> Date: Thu, 17 Dec 2015 10:06:13 -0800
> On Thu, Dec 17, 2015 at 7:27 AM, Edward Cree <ec...@solarflare.com> wrote: >> When the inner packet checksum is offloaded, the outer UDP checksum is easy >> to calculate as it doesn't depend on the payload (because the inner checksum >> cancels out everything from the inner packet except the pseudo header). >> Thus, transmit checksums for VXLAN (and in principle other encapsulations, >> but I haven't enabled it for / tested with those) can be offloaded on any >> device supporting NETIF_F_HW_CSUM. Only the innermost checksum has to be >> offloaded, the rest are filled in by the stack. >> Tested by hacking a driver to report NETIF_F_HW_CSUM, call skb_checksum_help >> before transmitting a packet, and not actually offload anything to the hw. >> In principle it should also be possible to apply this technique when the >> inner packet has been checksummed by software, but only if skb->csum_start >> and skb->csum_offset have been filled in to describe the inner checksum. >> However in this case it is easier to use skb->csum and skb->csum_start, as >> gso_make_checksum() already does - a similar but simpler technique. It's >> not clear to me where else this should be done, so this is out of scope for >> this patch series. >> > Edward, it took me a while to understand how this works, but this > really is an amazing trick! This implies that we don't need to worry > about HW support for offloading multiple checksums. > > I'm not sure that we need bits in VXLAN or any other encapsulation. It > should be sufficient in udp_set_csum that if we already have > CHECKSUM_PARTIAL that can always be used to do local checksum offload. > This is also should be independent as to whether the device does > NETIF_F_HW_CSUM or can offload NETIF_F_IP[V6]_CSUM for encapsulated > packets. > > It would be nice to have a more formal documentation also. This is a > very powerful mechanism but the math behind it and requirements are > subtle. I also think this is amazing, but should be generalized. Also the pseudo header bits should be optimized a bit, running skb_checksum_help() on a two byte part of the packet can certainly be improved for example. :-) -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
