[Qemu-devel] inconsistency between device traversal in qdev and legacy

[Peter Maydell <peter.maydell@linaro.org>]

If you have a model with more than one USB bus, and you
create a USB device on the command line without specifying
which bus to plug it into, QEMU will choose a different bus
depending on whether you use the legacy "-usbdevice keyboard"
or the qdev "-device usb-kbd".

This is because the legacy option finds the USB bus with
usb_bus_find(), which searches the 'busses' list, which is
populated by usb_bus_new(), with each new bus added to the
tail of the list. The qdev option looks for the USB bus
with qbus_find_recursive(), which walks the qdev tree.
Since qbus_create_inplace() adds each new child bus to
the front of the parent's child_bus list, this means that
qbus_find_recursive() will encounter the last-added bus
first, whereas usb_bus_find() will get the first-added bus.

I assume this is likely to apply to other bus types as well.

Is there anything we can do to fix this inconsistency [*],
or are we tied to the existing enumeration orders in both
cases for compatibility with users with currently-working
command lines or configurations?

[*] the actual code changes are simple enough, obviously

-- PMM