On Fri, Feb 17, 2017 at 1:04 PM, Tom Herbert <t...@herbertland.com> wrote: > Alexei and I were looking at the mlx5 LRO code this morning. One > discrepancy I noticed with GRO is that CHECKSUM_UNNECESSARY is set for > the assembled packet whereas in GRO we use CHECKSUM_PARTIAL. The > effect of using checksum-unnecessary is that the actual TCP checksum > in the packet is not correct for the packet. Since gso_type set this > probably is okay when packet is forwarding (checksums recomputed), but > this seems to make some assumptions about the meaning of > checksum-unnecessary. Looking if the rules of checksum in skbuff.h > we're no explicit in saying whether the checksum actually in the > packet must be correct. > > I think there's some possible ways to address this: > > 1) Allow actual checksum to be incorrect in packet when the packet is > gso. On TX checksum must always be computed then. > 2) Change the instances where driving is modifying a packet or > creating a new one as in LRO case to use CHECKSUM_PARTIAL. > 3) Modify the checksum in the packet so that it is correct. We do this > in nearly all other cases where we modify the packet (e.g. NAT). Would > be hard to do in LRO though. > On second thought, I don't think #1 is correct. For instance, if we ever applied this to UDP that would break checksum-unnecessary conversion. Also, the stack should always have the prerogative to not trust checksum-unnecessary mark and do the calculation itself. checksum-unnecessary should mean the checksum has been verified to be correct and it is correct in the packet.
For LRO, drivers should use checksum-partial I think. Tom > In any case, I think we need to update sk_buff.h to clarify what the > semantics are. > > Tom
