WinDbg causes Windows to bluescreen (0x00000124) at boot

I am running Windows 7 in debug mode inside a VirtualBox VM.

I have configured a COM port debug server on the VM target, which is exposed to the Windows 7 host as a named pipe.

The VM boots fine by itself, but when I try to boot with the debugger it causes the target to crash.

Here are the steps I am taking:

1. Launch WinDbg on the host.
2. Start the target VM instance.
3. WinDbg connects and the VM freezes at the "starting windows" screen.
4. At this point things start going wrong. WinDbg output reports fatal system error.
5. WinDbg waits at a breakpoint in the target.
6. "Resume" target (F5) causes target to bluescreen.

The full WinDbg session output is as follows. Any help appreciated!

Microsoft (R) Windows Debugger Version 6.12.0002.633 AMD64
Copyright (c) Microsoft Corporation. All rights reserved.

Waiting for pipe \\.\pipe\SlaveDebugPipe
Waiting to reconnect...
Connected to Windows 7 7601 x64 target at (Thu Feb  7 12:58:54.482 2013 (UTC + 0:00)), ptr64 TRUE
Kernel Debugger connection established.
Symbol search path is: srv*c:\symbols*http://msdl.microsoft.com/download/symbols
Executable search path is: 
Windows 7 Kernel Version 7601 MP (1 procs) Free x64
Built by: 7601.17944.amd64fre.win7sp1_gdr.120830-0333
Machine Name:
Kernel base = 0xfffff800`0265d000 PsLoadedModuleList = 0xfffff800`028a1670
System Uptime: not available

*** Fatal System Error: 0x00000124
                       (0x0000000000000000,0x0000000000000000,0x0000000000000000,0x0000000000000000)

Break instruction exception - code 80000003 (first chance)

A fatal system error has occurred.
Debugger entered on first try; Bugcheck callbacks have not been invoked.

A fatal system error has occurred.

Connected to Windows 7 7601 x64 target at (Thu Feb  7 12:59:48.476 2013 (UTC + 0:00)), ptr64 TRUE
Loading Kernel Symbols
..................................................
Loading User Symbols

*******************************************************************************
*                                                                             *
*                        Bugcheck Analysis                                    *
*                                                                             *
*******************************************************************************

Use !analyze -v to get detailed debugging information.

BugCheck 124, {0, 0, 0, 0}

Probably caused by : hardware

Followup: MachineOwner
---------

nt!DbgBreakPointWithStatus:
fffff800`026d3df0 cc              int     3

This problem is caused by the VM having been configured to use multiple CPU's.

Use the slider in the System panel of the VirtualBox instance configuration window to set the CPU count to 1.

After this the system will boot normally with the debugger attached.

This problem is discussed here.