Debugging windows EFI bootloader

[matusalem]

I recently have tried to debug bootmgfw.efi x64 version of Windows 8.
I use last debugging tools (Windbg x64) for Windows 8 from a windows 7 station (x64).
So i used bcdedit to enable bootdebug on {bootmgr} partition, serial as debug type, debugport 2 and baudrate at 115200.
Next i ran "windbg.exe -k com:port=\\.\pipe\kdebug,baud=115200,pipe,reconnect" command to connect to the debugger stub and i have this message:

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Microsoft (R) Windows Debugger Version 6.2.9200.16384 AMD64
Copyright (c) Microsoft Corporation. All rights reserved.

Opened \\.\pipe\kdebug
Waiting to reconnect...
Connected to Windows Boot Debugger 9200 x64 target at (Tue Oct  2 10:45:15.324 2012 (UTC + 2:00)), ptr64 TRUE
Kernel Debugger connection established.
Symbol search path is: srv*c:\symbols.pub*http://msdl.microsoft.com/download/symbols
Executable search path is: 
CS descriptor lookup failed
Windows Boot Debugger Kernel Version 9200 UP Free x64
Machine Name:
Primary image base = 0x00000000`10000000 Loaded module list = 0x00000000`10183280
System Uptime: not available
Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
Unable to get program counter
0018:942f 0000            add     byte ptr [bx+si],al

From here, i can't do anything: single step, continue running,...
Note that i have no problem to debug winload.exe by setting bootdebug mode on {current} partition, it works like a charm.
It seems that CS/SS segment values are wrong although CPU classical registers contains right values for RIP, RSP,...
Stack trace is working very well with all symbols handled but can't go further:

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kd> k
Child-SP          RetAddr           Call Site
00000000`60fcd848 00000000`10052aec bootmgfw!DbgBreakPointWithStatus
00000000`60fcd850 00000000`100529bc bootmgfw!vDbgPrintExWithPrefixInternal+0x128
00000000`60fcd940 00000000`10093011 bootmgfw!DbgPrint+0x20
00000000`60fcd970 00000000`10093670 bootmgfw!BlBdStart+0x141
00000000`60fcd9b0 00000000`10034323 bootmgfw!BlBdInitialize+0x388
00000000`60fcda50 00000000`100013d8 bootmgfw!BlInitializeLibrary+0x33
00000000`60fcda80 00000000`1000101e bootmgfw!BmMain+0x208
00000000`60fcdbe0 00000000`60fd10cf bootmgfw!EfiEntry+0x1e
00000000`60fcdc10 80000000`00000003 0x60fd10cf
00000000`60fcdc18 00000000`5ebe2c18 0x80000000`00000003
00000000`60fcdc20 00000000`00000000 0x5ebe2c18
kd> p
    ^ Unable to get program counter 'p'