From: Edward Cree <ec...@solarflare.com> Date: Mon, 7 Dec 2015 15:39:52 +0000
> 1) Receive checksums. Given that CHECKSUM_UNNECESSARY conversion > exists (and is a cheap operation), what is the advantage to the > stack of using CHECKSUM_COMPLETE if the packet happens to be a > protocol which CHECKSUM_UNNECESSARY conversion can handle? As I see > it, CHECKSUM_UNNECESSARY is strictly better as the stack is told > "the first csum_level+1 checksums are good" *and* (indirectly) "here > is the whole-packet checksum, which you can use to help with > anything beyond csum_level+1". Is it not, then, best for a device > only to use CHECKSUM_COMPLETE for protocols the conversion doesn't > handle? (I agree that having that fallback of CHECKSUM_COMPLETE is > a good thing, sadly I don't think our new chip does that. (But > maybe firmware can fix it.)) No, it is better to universally provide the 1's complement sum for all receive packets. This allows the stack more flexibility in checksum handling. > 3) Related to the above, what does a NETIF_F_HW_CSUM device do when > transmitting an unencapsulated packet (let's say it's UDP) > currently? Will it simply get no checksum offload at all? Will > csum_start point at the regular UDP checksum (and the stack will do > the IP header checksum)? Again, a device that does both HW_ and > IP_CSUM could cope with this (do the IP and UDP checksums as per > NETIF_F_IP_CSUM, and just don't ask for a 'generic' HW_CSUM), though > that would require more checksum flags (there's no way for > CHECKSUM_PARTIAL to say "do your IP-specific stuff but ignore > csum_start and friends). The stack will have skb->csum_start point to the UDP header's checksum field for unencapsulated packets, and it has done this for decades. Sun Microsystems had NETIF_F_HW_CSUM supporting NICs nearly two decades ago, and this is what NETIF_F_HW_CSUM was designed for. -- 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
