Re: kvm guest debug using gdb on x86

[Jan Kiszka <jan.kiszka@siemens.com>]

Aneesh Kumar K.V wrote:
> On Wed, May 20, 2009 at 10:57:21AM +0200, Jan Kiszka wrote:
>> Aneesh Kumar K.V wrote:
>>> On Wed, May 20, 2009 at 10:10:59AM +0200, Jan Kiszka wrote:
>>>> Aneesh Kumar K.V wrote:
>>>>> On Wed, May 20, 2009 at 12:23:12AM +0530, Aneesh Kumar K.V wrote:
>>>>>> Hi,
>>>>>>
>>>>>> With the latest qemu-kvm and 2.6.30-rc6 kernel i am not able to get
>>>>>> the guest debugging with gdb. I get the following error.
>>>>>>
>>>>>> $gdb ./vmlinux
>>>>>> GNU gdb 6.8-debian
>>>>>> Copyright (C) 2008 Free Software Foundation, Inc.
>>>>>> License GPLv3+: GNU GPL version 3 or later
>>>>>> <http://gnu.org/licenses/gpl.html>
>>>>>> This is free software: you are free to change and redistribute it.
>>>>>> There is NO WARRANTY, to the extent permitted by law.  Type "show
>>>>>> copying"
>>>>>> and "show warranty" for details.
>>>>>> This GDB was configured as "i486-linux-gnu"...
>>>>>> (gdb) b do_fork
>>>>>> Breakpoint 1 at 0xc106cfc8: file kernel/fork.c, line 1347.
>>>>>> (gdb) target remote localhost:1234
>>>>>> Remote debugging using localhost:1234
>>>>>> [New Thread 1]
>>>>>> Remote 'g' packet reply is too long:
>>>>>> 7fa5000000000000000000000000000057e209c10000000004000000000000000000000000000000c8b3d0c100000000c03fd1c100000000a83fd1c10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000912d03c1000000000202000060000000680000007b0000007b000000d8000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000f60b801540000000000000000000007f030000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 
>  000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
>> 00
>>>> 0000000000000000000000000000000000000000000000000000000000000000000000000000000000000
>>>>>> (gdb)
>>>>>>
>>>>>> any patches that i can try ?
>>>>> Works better with the four patches found at
>>>>>
>>>>> http://git.kiszka.org/?p=kvm-userspace.git;a=shortlog;h=refs/heads/queues/gdb
>>>>>
>>>>> But a next and continue doesn't get the prompt back on gdb. The guest
>>>>> does stops the execution.
>>>> What arch is host and guest (x86-32 or -64)? What KVM versions are you
>>>> using? Are you sure that guest and vmlinux matches?
>>>>
>>> Both host and guest are x86-32.  For kvm user space I did a git clone
>>> from git://git.kernel.org/pub/scm/virt/kvm/qemu-kvm.git. I tested with
>>> commit 22d239bcee126742df46938ee8ddc7c6b9209e23. That version fails
>>> with the "remote g packet reply is too long" error. Then i applied the
>>> four patches from your git repo. That gets the guest to stop execution
>>> on breakpoint, i can check the stack. but single stepping doesn't work.
>>>
>>> Guest kernel I tried was linux-2.6-tip
>> Does debugging work with -no-kvm, ie. in TCG mode?
> 
> 
> Yes. debugging works with -no-kvm option. I can do single stepping.
> 

Sigh, confirmed, there is a regression in qemu-kvm. Will dig into this.
Older kvm-userspace should be fine (e.g. the last version you can pull
from my tree).

Jan

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