On Fri, May 4, 2018 at 1:19 PM, Eric Dumazet <eric.duma...@gmail.com> wrote: > > > On 05/04/2018 11:30 AM, Alexander Duyck wrote: >> From: Alexander Duyck <alexander.h.du...@intel.com> >> >> This patch is meant to allow us to avoid having to recompute the checksum >> from scratch and have it passed as a parameter. >> >> Instead of taking that approach we can take advantage of the fact that the >> length that was used to compute the existing checksum is included in the >> UDP header. If we cancel that out by adding the value XOR with 0xFFFF we >> can then just add the new length in and fold that into the new result. >> > >> >> + uh = udp_hdr(segs); >> + >> + /* compute checksum adjustment based on old length versus new */ >> + newlen = htons(sizeof(*uh) + mss); >> + check = ~csum_fold((__force __wsum)((__force u32)uh->check + >> + ((__force u32)uh->len ^ 0xFFFF) + >> + (__force u32)newlen)); >> + > > > Can't this use csum_sub() instead of this ^ 0xFFFF trick ?
I could but that actually adds more instructions to all this since csum_sub will perform the inversion across a 32b checksum when we only need to bitflip a 16 bit value. I had considered doing (u16)(~uh->len) but thought type casing it more than once would be a pain as well. What I wanted to avoid is having to do the extra math to account for the rollover. Adding 3 16 bit values will result in at most a 18 bit value which can then be folded. Doing it this way we avoid that extra add w/ carry logic that is needed for csum_add/sub.
