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Re: Mac OS real USB device support issue

From: BALATON Zoltan
Subject: Re: Mac OS real USB device support issue
Date: Wed, 7 Apr 2021 01:18:51 +0200 (CEST)

On Tue, 6 Apr 2021, Programmingkid wrote:
On Apr 6, 2021, at 12:53 PM, BALATON Zoltan <balaton@eik.bme.hu> wrote:
On Tue, 6 Apr 2021, Programmingkid wrote:
On Apr 6, 2021, at 10:01 AM, Howard Spoelstra <hsp.cat7@gmail.com> wrote:
On Tue, Apr 6, 2021 at 3:44 PM Programmingkid <programmingkidx@gmail.com> wrote:

Hi Gerd,

I was wondering if you had access to a Mac OS 10 or Mac OS 11 machine to test 
USB support. I am on Mac OS 11.1 and cannot make USB devices work with any of 
my guests. So far these are the guests I have tested with:

- Windows 7
- Mac OS 9.2
- Windows 2000

I have tried using USB flash drives, USB sound cards, and an USB headset. They 
all show up under 'info usb', but cannot be used in the guest. My setup does 
use a USB-C hub so I'm not sure if this is a bug with QEMU or an issue with the 
hub. Would you have any information on this issue?

Hi John,

As far as the Mac OS 9.2 guest is concerned on a mac OS host, it does
not support USB 2.0. I was successful only in passing through a USB
flash drive that was forced into USB 1.1 mode by connecting it to a
real USB 1.1 hub and unloading the kext it used.


Hi Howard, I was actually thinking about CC'ing you for this email. Glad you 
found it. Unloading kext files does not sound pleasant. Maybe there is some 
better way of doing it.

In any case, until you make sure nothing tries to drive the device on the host, 
passing it to a guest likely will fail because then two drivers from two OSes 
would try to access it simultaneously which likely creates a mess as the device 
and drivers don't expect this. So you can't just pass a device through that the 
host has recognised and is driving without somehow getting the host to leave it 
alone first before you can pass it through. Unloading the driver is one way to 
do that (although it probably breaks all other similar devices too). Maybe 
there's another way to unbind a device from the host such as ejecting it first 
but then I'm not sure if the low level USB needed for accessing the device 
still works after that or it's completely forgotten. There's probably a doc 
somewhere that describes how it works and how can you plug a device without 
also getting higher level drivers to load or if there's no official ways for 
that then you'll need to do some configuration on the host t
o avoid it grabbing devices that you want to pass through. On Linux you can add an udev 
rule to ignore the device (maybe also adding TAG+="uaccess" to allow console 
users to use it without needing root access) but not sure how USB works on macOS.


Being able to dissociate a real USB device from its Mac OS driver would be very useful in this situation. IOKit might be one place to look for such a feature. The Mach kernel documentation is another place that might have what we want.

Those might be a good place to start. IOKit provides the drivers and also the io registry which is probably where you can get if a driver is bound to a device and which one is it. How to dissociate the driver from the device though I don't know.

I have one theory. What if we introduce a middleman. A pseudo-USB device that the guest operating system could apply its configuration data to and will also talk directly with to the real USB device.
So this:

USB device <-> Host <-> QEMU USB middleman <-> Guest

Isn't this middleman the QEMU usb-host device that we already have?

This could make USB 2.0 and 3.0 flash drives compatible with an older operating system like Mac OS 9. The USB middleman could fully accept Mac OS 9's configuration and make it think it is talking to a USB 1.1 device. Parameters like data packet payload size would no longer be a problem. Host driver unloading would no longer be needed (in theory).

However I think you're mixing up a few things here. The idea of passing through USB devices is to let the guest handle it with its own drivers like it was connected directly to the virtual machine and not to emulate a USB device using host resources. If you want the latter then use usb-storage, usb-audio or similar. All that usb-host does is just forwarding the packets from guest to the physical device and let it talk to it and drive it without help from the host. (I may be wrong about the details, I haven't checked actual code but at least conceptually this should be the case.) If you have this scenario then you can easily see that both the host and guest driving the same USB device will not work. You should not try to mount a USB drive in guest that's already mounted by the host or you can't send audio from both the guest and the host at the same time without totally confusing the device and both drivers that don't expect this to ever happen. So for passing through the device you have to make sure the host does not try to access it while it's used by the guest.

If your guest does not have drivers for the device that you want to pass through that's a different problem. With pass through the guest is exclusively given the task of driving the device so it should have a driver for it. If the device does not work with the guest if you plug it in a physical machine then it won't work with pass through either. But the problem here is probably not that but the disagreement between USB speed between host and guest. If the guest does not have USB 2.0 then you can't pass through USB 2.0 devices unless downgrading them on the host as well in some way. So you either connect them to an USB 1 hub to match the emulated USB hardware in the guest or you need to emulate an USB 2 card in the guest and connect passed through devices to that. Did USB 2 cards exist for older G3/G4 Macs? Some PowerBooks had USB 2 ports, what hardware did those use? Could those be emulated in QEMU? These are separate problems though from getting the device freed from host drivers to avoid the problems with both guest and host accessing the device.


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