Problems with debugging over USB 3.0 cable

I cannot make kernel debugging over USB 3.0 work. The debugging session starts, some communication happens, but then it gets stuck.

Windows 8.1 is installed on both target and host computers. WDK for Windows 8.1 is installed on the host computer. The target computer has been configured for debugging per , its USB 3.0 controller is debug-capable (RENESAS UPD720202).

Command line:
“windbg.exe /k usb:targetname=foo -y srv*C:\Symbols*
produces the following output.

Microsoft (R) Windows Debugger Version 6.3.9600.17237 AMD64
Copyright (c) Microsoft Corporation. All rights reserved.

Using USB for debugging
Waiting to reconnect…
USB: Write opened
Connected to Windows 8 9600 x64 target at (Sun Sep 14 15:40:42.520 2014 (UTC + 2:00)), ptr64 TRUE
Kernel Debugger connection established.

************* Symbol Path validation summary **************
Response Time (ms) Location
Deferred srv*C:\Symbols*
Symbol search path is: srv*C:\Symbols*
Executable search path is:
Windows 8 Kernel Version 9600 MP (1 procs) Free x64
Product: WinNt, suite: TerminalServer SingleUserTS
Built by: 9600.16384.amd64fre.winblue_rtm.130821-1623
Machine Name:
Kernel base = 0xfffff80265a81000 PsLoadedModuleList = 0xfffff80265d489b0

As you can see, the debugger receives the target OS version, so some communication happens, but that’s all. The target computer remains at the boot logo screen despite long waiting. Of course, I’ve made sure it is not a breakpoint :).

There was also a 3 minute delay between “Executable search path is:” line and the next lines. Looks like the communication was very slow. But USB 3.0 debugging is normally very fast, right?

Another strange thing is that Device Manager shows the target computer as “USB 2.0 Debug Connection Device” with a driver named “usb2dbg.sys”. It cannot really be USB 2.0, because I have a USB 3.0 debug cable with USB 2.0 pins 1,2,3 not connected at all! Is it normal? (Maybe usb2dbg.sys is also used for 3.0 mode…)