(Discussed) Usb versions selection

[makh]

Dear Admin

These days its a normal that some usb devices are 2.0 or 3.0 compliant.

On the fly we cannot attach a different device, machine has to be shutdown, then change compatibility, then start machine to use a different device.

Why is this feature so complicated?

Thankyou

[mpack]

It is not complicated. Configure the VM for USB3. USB3 includes backwards compatibility with USB2.

[michaln]

USB3 includes backwards compatibility with USB2.

Actually, it doesn't, USB3 is a separate standard from USB2. Both USB2 and USB3 standards are current. But the xHCI controller does handle both, which is probably the only thing users care about.

There are two major reasons why those selections exist in the VirtualBox GUI. One is that USB2/3 functionality is delivered in the Extension Pack, separate from the base VirtualBox package. So users may not have it. The other reason is that not all operating systems support USB3, notably Windows 7. If USB3 is selected for a vanilla Windows 7 VM, there will be no USB at all. That's why the feature is so complicated.

[mpack]

The USB3 standard itself doesn't mandate backwards compatibility with USB2 devices? That's interesting - I thought it did. Perhaps as you say I've been reading about what PCs do (i.e. typically xHCI), instead of what USB3 itself does.

[michaln]

There are two separate things. If you look at the USB3 specification, you will find that it is quite different from the USB2 specification. The electrical layer is completely different, the wiring is different, the device protocol is similar but not the same.

The USB3 port is actually two ports in one, USB2 and USB3. Similarly a USB3 hub is really two in one, USB3 and USB2. A USB3 device has two interfaces, USB2 and USB3. And likewise a xHCI controller has separate USB2 and USB3 ports.

USB3 hosts and devices are required to support USB2 as well, so from a user's point of view the technologies are interoperable. On the lower level USB2 and USB3 live side by side but are quite separate.

Quite a few systems have separate USB3 (i.e. USB3 + USB2) and USB2-only ports, often driven by different controllers. So even for a USB2 device, it can make a difference what port the device is plugged into.

Is it a big mess? Yes, it is.

[michaln]

The USB 3.0 specification says: "USB 3.0 utilizes a dual-bus architecture that provides backward compatibility with USB 2.0. It provides for simultaneous operation of SuperSpeed and non-SuperSpeed (USB 2.0 speeds) information exchanges." That is hidden from users when everything works 100%. When it doesn't work... the ugly guts start poking out.

[makh]

So the usb is getting complicated for no reason. It doesnt sounds nice if it lacks backward compatibility.

[michaln]

But it doesn't. It's just that "backwards compatibility" means keeping the existing USB2 infrastructure more or less as is, and adding USB3 alongside it.