[Bug 1843711] Re: qemu-xhci device should detect if libusb host supports streams

[Anisse Astier]

This is a suggestion that would really help anyone using trying to use
xhci passthrough on a platform without streams.

** Changed in: qemu
       Status: Incomplete => New

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https://bugs.launchpad.net/bugs/1843711

Title:
  qemu-xhci device should detect if libusb host supports streams

Status in QEMU:
  New

Bug description:
  When using USB passthrough with the qemu-xhci (and nec-usb-xhci),
  streams are enabled by default, but if the host xHCI controller
  doesn't support them, it will trigger hard-to-debug UAS guest errors.

  This should be possible to detect since the kernel returns ENOSYS (errno 38) 
when xhci host controller does not support streams:
              libusb: error [do_streams_ioctl] streams-ioctl failed error -1 
errno 38
              qemu: libusb_alloc_streams: -99 [OTHER]

  Maybe libusb should return a dedicated error instead of
  LIBUSB_ERROR_OTHER in this case, but qemu does not handle any other
  error code anyway.

  Just trying to enable streams before enabling them in qemu should do
  it. I don't know if it should be done in hcd-xhci.c, host-libusb.c or
  elsewhere, but this would be detectable at launch instead of a static
  option true/false, maybe a ternary with auto would be better.

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