Emulated eXtensible Host Controller Interface (xHCI) host controller

[lifning]

Special thanks to @luqmana getting this started in early 2023: https://github.com/luqmana/propolis/commits/xhci/

(commit-message design outline that follows can also be found in cargo doc at /propolis/hw/usb/xhci/index.html, with the named identifiers being hypertext)

---

The version of the standard referenced throughout the comments in this module is xHCI 1.2, but we do not implement the features required of a 1.1 or 1.2 compliant host controller - that is, we are only implementing a subset of what xHCI version 1.0 requires of an xHC, as described by version 1.2 of the specification.

https://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/extensible-host-controler-interface-usb-xhci.pdf

At present, the only USB device supported is a USB 2.0 NullUsbDevice with no actual functionality, which exists as a proof-of-concept and as a means to show that USB DeviceDescriptors are successfully communicated to the guest in phd-tests.

   +---------+
   | PciXhci |
   +---------+
        | has-a
  +-----------------------------+
  |          XhciState          |
  |-----------------------------|
  | PCI MMIO registers          |
  | XhciInterrupter             |
  | DeviceSlotTable             |
  | Usb2Ports + Usb3Ports       |
  | CommandRing                 |
  | newly attached USB devices  |
  +-----------------------------+
      | has-a               |
+-------------------+       | has-a
|  XhciInterrupter  |   +-----------------+
|-------------------|   | DeviceSlotTable |
| EventRing         |   |-----------------|
| MSI-X/INTxPin     |   | DeviceSlot(s)   |___+------------------+
+-------------------+   | DCBAAP          |   |    DeviceSlot    |
                        | Active USB devs |   |------------------|
                        +-----------------+   | TransferRing(s)  |
                                              +------------------+

Conventions

Wherever possible, the framework represents Trb data through a further level of abstraction, such as enums constructed from the raw TRB bitfields before being passed to other parts of the system that use them, such that the behavior of identifying TrbType and accessing their fields properly according to the spec lives in a conversion function rather than strewn across implementation of other xHC functionality.

The nomenclature used is generally trading the "Descriptor" suffix for "Info", e.g. the high-level enum-variant version of an EventDescriptor is EventInfo (which is passed to the EventRing to be converted into Event TRBs and written into guest memory).

For 1-based indeces defined by the spec (slot ID, port ID), we put placeholder values at position 0 of any arrays in which the ID is used as an index, such that we aspire to categorically avoid off-by-one errors of omission (of - 1).

Implementation

DeviceSlotTable

When a USB device is attached to the xHC, it is enqueued in a list within XhciState along with its PortId. The next time the xHC runs:

• it will update the corresponding PORTSC register and inform the guest with a TRB on the EventRing, and if enabled, a hardware interrupt.
• it moves the USB device to the DeviceSlotTable in preparation for being configured and assigned a slot. When the guest xHCD rings Doorbell 0 to run an EnableSlot Command, the DeviceSlotTable assigns the first unused slot ID to it.

Hot-plugging devices live (i.e. not just attaching all devices defined by the instance spec at boot time as is done now) is not yet implemented.

Device-slot-related Command TRBs are handled by the DeviceSlotTable. The command interface methods are written as translations of the behaviors defined in xHCI 1.2 section 4.6 to Rust, with liberties taken around redundant TrbCompletionCode writes; i.e. when the outlined behavior from the spec describes the xHC placing a Success into a new TRB on the EventRing immediately at the beginning of the command's execution, and then overwriting it with a failure code in the event of a failure, our implementation postpones the creation and enqueueing of the event until after the outcome of the command's execution (and thus the Event TRB's values) are all known.

Ports

Root hub port state machines (xHCI 1.2 section 4.19.1) and port registers are managed by Usb2Port, which has separate methods for handling register writes by the guest and by the xHC itself.

TRB Rings

Consumer:

The CommandRing and each slot endpoint's TransferRing are implemented as ConsumerRing<CommandInfo> and
ConsumerRing<TransferInfo>. Dequeued work items are converted from raw CommandDescriptors and TransferDescriptors, respectively).

Starting at the dequeue pointer provided by the guest, the ConsumerRing will consume non-Link TRBs (and follow Link TRBs, as in xHCI 1.2 figure 4-15) into complete work items. In the case of the CommandRing, CommandDescriptors are each only made up of one Trb, but for the TransferRing multi-TRB work items are possible, where all but the last item have the chain_bit set.

Producer:

The only type of producer ring is the EventRing. Events destined for it are fed through the XhciInterrupter, which handles enablement and rate-limiting of PCI-level machine interrupts being generated as a result of the events.

Similarly (and inversely) to the consumer rings, the EventRing converts the EventInfos enqueued in it into EventDescriptors to be written to guest memory regions defined by the EventRingSegment Table.

Doorbells

The guest writing to a DoorbellRegister makes the host controller process a consumer TRB ring (the CommandRing for doorbell 0, or the corresponding slot's TransferRing for nonzero doorbells). The ring consumption is performed by the doorbell register write handler, in process_command_ring and process_transfer_ring.

Timer registers

The value of registers defined as incrementing/decrementing per time interval, such as MFINDEX and the XhciInterrupter's IMODC, are simulated with Instants and Durations rather than by repeated incrementation.

DTrace support

To see a trace of all MMIO register reads/writes and TRB enqueue/dequeues:

pfexec ./scripts/xhci-trace.d -p $(pgrep propolis-server)

The name of each register as used by DTrace is &'statically defined in registers::Registers::reg_name.