USB 3 for vUSB

[stormi]

QEMU supports USB3 emulation, but XAPI only offers USB2 . We made a test with a USB storage device, and letting XAPI ask for xhci instead of ehci for high speed devices worked very well. Transfert speed for a mass storage device went from 15MB/s to 100MB/s.

I'm opening this issue so that we can discuss before submitting a design.

There's already code in XAPI that selects either USB1 (usb-bus) or USB2 (ehci), depending on device speed. It would be trivial to make XAPI select xhci when speed is above a certain threshold, but I'm no USB expert and I suspect maybe it's not that simple. However, QEMU doc says xhci can handle all cases, so maybe we should always use xhci.

Comes also the topic of the transition for existing VMs. I suppose changing the emulation type will change the devices exposed to the VM, so we probably can't change it for existing VMs without letting users explicitly select it?

A possible way forward:

• Select xhci by default for high speed devices from now on, for new VMs.
• Allow users to change the setting via XAPI calls: back to ehci in case there's an issue, or forward to xhci for existing VMs for which they need the performance boost.
• Also allow users to set it when they add a vUSB to a VM.
• At some point, consider using xhci by default whatever the speed?

We also need to evaluate the security aspects of xhci emulation in the current versions of QEMU in XenServer and XCP-ng, I suppose.

@gthvn1 could you link towards the related code and towards the QEMU doc that says xhci can handle all cases?

[gthvn1]

Yes sure.
The code that chooses the usb type based on the detected speed is here in vusb_plug.

About the qemu doc there is a dedicated part about USB where we can read in the first paragraph (or at least one of the first :) )

XHCI supports USB 1.1, USB 2.0 and USB 3.0 devices, so this is the only controller you need. With only a single USB controller (and therefore only a single USB bus) present in the system there is no need to use the bus= parameter when adding USB devices.

[psafont]

I have two notes here:

• Guests might not support it, so it makes sense that using xhci would be gated in some way for guests (and not just existing ones).
• xhci supports all speeds, so I would prefer to remove xhci from the speed equation. That is, it's selected whenever it's not being gated.

[gthvn1]

So if I understand correctly we could add on option per VM to enable/disable xhci. If enabled then always use it, otherwise use the current behavior that is either USB1 (usb-bus) or USB2 (ehci) based on the speed. And by default the option can be set to disabled that means by default we keep the current behavior.

[psafont]

Yes, probably a per-pool, or a per-host setting would make sense here.

You probably want to change the default at some point. Maybe this can be changed using a .conf in xapi so it can be changed on an update when deemed safe by xcp-ng, or xenserver

[stormi]

One says per VM, one says per host/pool :)