For a while, I've been working towards consolidation of machines into VMs on my unRAID system. unRAID is great for this as it allows for hardware passthrough of devices such as GPUs, USB adapters and other items. Passthrough of discrete USB controllers is especially important. unRAID does allow direct device passthrough, but I've found this unsatisfactory for a number of reasons. Mainly, it's not a great solution for high-bandwidth devices such as audio cards. Also, it's not really plug'n'play, with hot swapping being somewhat patchy.Passthrough of the full controller solves these problems, One of the challenges of this approach is however, physical capacity for discrete devices. With each VM requiring a PCIe GPU and a USB adapter, it's possible to run out of slots fairly quickly. Yes, it's possible to use on-board controllers, but that's not always a full solution and PCIe cards are often required in addition.
One such card is the Sonnet Allegro USB3-PRO-4PM-E, sadly now discontinued. There are alternatives, but as users in this thread are finding out, broad compatibility is patchy.
As an alternative, I've been using a PCIe expansion kit to pass through discrete controllers to VMs. These devices were all the rage during the mining boom as they allowed users attach multiple GPUs to a single PCIe slot in order to leverage compute power. They work by essentially switching multiple PCIe ports through a single 1x motherboard port. Although the device sports four 16x mechanical slots, each is in itself only a 1x slot - but that's all that's needed for a USB controller.
