On Fri, Apr 30, 2021 at 02:36:49PM +0300, Pavel Balaev wrote: > On Thu, Apr 29, 2021 at 06:29:20PM +0300, Ido Schimmel wrote: > > On Wed, Apr 28, 2021 at 03:31:33PM +0300, Pavel Balaev wrote: > > This looks overly complex to me and I believe a lot of users will ask > > themselves why they need to specify a seed using two hex numbers > > separated by a comma. Looking at other implementations that already > > allow specifying the seed, it is specified as a single integer. > > > > 32-bit in Cumulus: > > https://docs.nvidia.com/networking-ethernet-software/cumulus-linux-43/Layer-3/Routing/Equal-Cost-Multipath-Load-Sharing-Hardware-ECMP/#configure-a-hash-seed-to-avoid-hash-polarization > > > > Up to 16-bit in Arista: > > https://eos.arista.com/hashing-for-l2-port-channels-and-l3-ecmp/ > > > > I believe you chose this interface because of the structure of the > > SipHash key that is used for the multipath hash calculation. This is an > > internal implementation detail and should not determine the user > > interface. > > > > Looking at the history of the code, the flow dissector was migrated to > > SipHash in commit 55667441c84f ("net/flow_dissector: switch to > > siphash"). The motivating use case was flow label generation since these > > are sent on the wire together with the fields from which they were > > computed, not multipath hash calculation that also happens to rely on > > the flow dissector. > > > > Given the above, do you see a problem with having the user specify a > > 32-bit number for the multipath hash seed? Note that SipHash is still > > used and that the number can be used to fill the entire 128-bit space. > > Do you mean take 32-bit number from user and multiply it like this: > u32 key = val; > u64 key64; > memset(&key64, val, sizeof(u32)); > memset(&key64 + sizeof(u32), val, sizeof(u32)); > memset(seed.key[0], &key64, sizeof(u64)); > memset(seed.key[1], &key64, sizeof(u64)); > ? Something like that, yes. It's still only 32-bit of user input, but it can't hurt. Do you see a need to specify more than 32 bits for multipath hash seed when the purpose is to force the same seed on multiple machines?
