gdbserver-xen x86_64 paravirt guest debugging

[Ryan Harper <ryanh@us.ibm.com>]

I've attempted to debug live x86_64 domU domains with little success.
gdbserver-xen segfaults and I've starting running gdb on gdbserver-xen
to see where things are going south.

I kick off the server under gdb, and then run gdb client and remote
attach.  This appears to succeed, and gives me:

[New Thread 0]
[Switching to Thread 0]
0xffffffff8014e258 in softlockup_tick (regs=0xffff880026c6fcd8) at
kernel/softlockup.c:50
50              unsigned long timestamp = per_cpu(timestamp, this_cpu);

But when I ask for a backtrace:

(gdb) bt
#0  0xffffffff8014e258 in softlockup_tick (regs=0xffff880026c6fcd8) at
kernel/softlockup.c:50
Ignoring packet error, continuing...
Reply contains invalid hex digit 116


On the other side (gdbserver-xen) I see:

(gdb) run
The program being debugged has been started already.
Start it from the beginning? (y or n) y

Starting program: /home/rharper/work/openhype/xen/unstable/hg/d/tools/debugger/gdb/gdb-6.2.1-linux-x86_64-xen/gdb/gdbserver/gdbserver-xen 127.0.0.1:9999 --attach 1
domain currently paused
Attached; pid = 1
Listening on port 9999
Remote debugging from host 127.0.0.1

Program received signal SIGSEGV, Segmentation fault.
0x00002b2611bf410a in map_domain_va_64 (xc_handle=7, cpu=0, guest_va=0xffffffff80364ed0,
    perm=1) at xc_ptrace.c:295
295         l3p = page_array[l3p];

Some inspection:

(gdb) p l3p
$4 = 796261
(gdb) p nr_pages
$5 = 196608

page_array only has 196k entries AFAICT, so this seems problematic.

Anyone have x86_64 paravirt guest debugging working on latest unstable?

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
(512) 838-9253   T/L: 678-9253
ryanh@us.ibm.com