Thank you for the clarifying question. I had been trying to gather information through my (single) sys-usb VM before making any configuration changes due to the sensitive nature of the configuration. Based on your question I decided that I needed to actually try it in order to get enough information to get help, which led me to discover that my hardware does not support the configuration I am trying to create. I have written details below in case it is helpful to others.
In sys-usb, it is not true that each USB Bus corresponds to exactly 1 physical controller. One of the busses is an emulated bus used to communicate pointer information. Additionally, it is possible for a single USB controller to appear as multiple busses. It looks like this is how the kernel manages the difference between USB 2.0 and 3.0 devices, so I would assume that this could also happen in a dom0 with no sys-usb, but I have not confirmed this.
On my machine, the emulated bus is consistently bus 1. It has a device named “Adomax Technology Co., Ltd QEMU Tablet” attached.
In order to get information about the number of physical controllers I had, I read the links of all of the usb controllers. Modifying the instructions at How to use USB devices | Qubes OS
$ for file in /sys/bus/usb/devices/usb*; do
readlink $file
done
../../../devices/pci0000:00/0000:00:05.0/usb1
../../../devices/pci0000:00/0000:00:06.0/usb2
../../../devices/pci0000:00/0000:00:06.0/usb3
Both usb2 and usb3 link to device 00:06.0, meaning they are communicating with the same USB controller. As it turns out, my machine only has a single physical controller, meaning that it is not possible for me to isolate the sensitive devices from other devices as I wanted, since their data has to be processed by the same VM (sys-usb).
To clarify my starting point, here is the terminal output which prompted my original problem description. All commands are run on sys-usb as created by the installer.
No USB devices attached:
$ lsusb
Bus 003 Device 002: ID 05e3:0749 Genesys Logic, Inc. SD Card Reader and Writer
Bus 003 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 0627:0001 Adomax Technology Co., Ltd QEMU Tablet
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Mouse attached, third item in the list. Note that it is on bus 2:
$ lsusb
Bus 003 Device 002: ID 05e3:0749 Genesys Logic, Inc. SD Card Reader and Writer
Bus 003 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 002 Device 004: ID 045e:07a5 Microsoft Corp. Wireless Receiver 1461C
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 0627:0001 Adomax Technology Co., Ltd QEMU Tablet
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Flash drive attached, second item in the list. Note that it is on bus 3:
Bus 003 Device 002: ID 05e3:0749 Genesys Logic, Inc. SD Card Reader and Writer
Bus 003 Device 003: ID 13fe:6300 Phison Electronics Corp. SP Mobile C31 (64GB)
Bus 003 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 0627:0001 Adomax Technology Co., Ltd QEMU Tablet
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub