Is it so unusual to have an XHCI that has only USB3 ports?
Yes, this is the first one I've heard of.
My understanding was that a port can either be USB3 or USB2 (assigned via the Supported Protocol Capability).
Each USB3 host connector has both a USB3 and USB2 port in the Supported Protocol Capability. See xHCI specification 7.2.2.1 "USB Protocols" note: "Note: To support USB3 device certification requirements for USB2 user attached devices, USB 2.0 and USB 3.x Supported Protocol Capabilities shall be declared if any USB3 connectors are associated with xHCI Root Hub ports that enable user attached devices. Refer to sections 11.1 and 11.3 in the USB3 spec"
This would mean that in order to work correctly with Linux, all XHCIs right now would have to support at least one USB2 port in addition to their USB3 ports.
Yes, that is currently the case. But normally each USB3 connector has a matching USB3 and USB2 port pair. USB 3.2 specification section 11.1 dictates that: 11.1 USB 3.2 Host Support for USB 2.0 "USB 3.2-capable ports on hosts shall also support USB 2.0 operation in order to enable backward compatibility with USB 2.0 devices. It should be noted, however, that USB 3.2-capable hosts are not required to support Enhanced SuperSpeed operation on all of the ports available on the host, i.e., some USB 3.2-capable hosts may have a mix of USB 2.0-only and USB 3.2-capable ports." (Similar statement exists in older USB 3.0 specification) xHC with just one USB3 port could maybe be possible in some built in SSIC device case without user attachable ports, or some setup with several host controllers where one handles the USB3 traffic and the other the USB2 traffic of the same USB3 connector. Thanks Mathias
