What's the feature?

OVS currently supports 16 general 32 bit registers (these can also be used as 8 x 64bit xregs or 4 x 128bit xxregs):
https://github.com/openvswitch/ovs/blob/5be77f1ecc521174af188ce9a9372c89db0d22e8/include/openvswitch/flow.h#L33
 

Why is it needed?

OVN uses OVS registers in its logical packet processing pipeline. And it's running out of registers to use. In order to avoid complex solutions that require pushing/popping registers to the stack it would be beneficial to increase the number of registers OVS supports (maybe double the amount?).
 

Who will benefit? 

OVN (maybe other controllers too).

Note: it might make sense for OVN devs to work on this task so opening it for component "OVN" for now.