Prevent calling USB backends multiple times.

[grehan]

Summary

The TRB processing loop could potentially call a back-end twice
with the same status transaction. While this was generally benign,
some code paths in the tablet backend weren't set up to handle
this case, resulting in a NULL dereference.

Fix by

• returning a STALL error when an invalid request was seen in the backend
• skipping a call to the backend if the number of packets in a status transaction was zero (this code fragment was taken from the Intel ACRN xhci backend)

The NULL dereferences will be fixed in a follow-up commit.

Discussed with/Reviewed by: Leon Dang (author)

Test Plan

Boot up a Linux guest with XHCI table configured; run 'lsusb -v' and
verify command complets.

Boot Linux/Windows guests in graphics mode with XHCI tablet and verify
mouse works as expected.

[jhb]

usr.sbin/bhyve/pci_xhci.c

1863

So if I'm understanding correctly, STALL means "non-fatal error, return partial results from previous completed commands" whereas IOERROR is supposed to mean "the entire transaction failed without any results"?

[grehan]

usr.sbin/bhyve/pci_xhci.c

1863

For control transfers that's really the only signalled error return e.g. for unimplemented requests to the device ("9.2.7 Request Error" in the USB3.0 spec, and also qemu backend code).

IOERROR would be if a device goes dead during a transfer.

These aren't currently propagated but I'm hoping to do that in an upcoming rework.