> But what about builds that combine 8 of the 5090 with infiniband between boxes?

Why Infiniband ("IB")? If it's for RDMA, that is possible with certain Ethernet cards/chipsets as well. Certainly Mellanox, but Broadcom:

* https://techdocs.broadcom.com/us/en/storage-and-ethernet-con...

and Intel as well:

* https://www.intel.com/content/www/us/en/support/articles/000...

Link level flow control or priority flow control needs to be supported on the switch ports as well.