Re: [Qemu-devel] Fwd: Guest VM debug (Int 3 panic)

[Jan Kiszka]

On 2013-09-26 16:14, Hu Yaohui wrote:
> Hi Jan,
> Thanks for your reply.
> On Thu, Sep 26, 2013 at 2:08 AM, Jan Kiszka <address@hidden> wrote:
> 
>> On 2013-09-25 20:08, Hu Yaohui wrote:
>>> Hi All,
>>> I am trying to debug guest OS through qemu with kvm enabled.
>>> Following is what I have done:
>>> 1: fire the qemu-kvm
>>> <snip>
>>> sudo qemu-system-x86_64 -hda vdisk.img -m 4096 -smp 2 -vnc :2 -boot c -s
>>> </snip>
>>>
>>> 2: wait until login into guest OS (ubuntu 10.04)
>>>
>>> 3: fire gdb
>>> <snip>
>>> gdb vmlinux
>>> target remote :1234
>>> b do_fork
>>> set arch i386:x86-64
>>
>> "set arch" is unneeded. vmlinux already tells gdb that you are debugging
>> x86-64.
>>
>>> c
>>> </snip>
>>>
>>> 4: after I typed "ls" in guest OS. The guest OS paniced with some message
>>> related to "int 3 blah blah". Then crashed.
>>>
>>> Someone said we should use hardware breakpoint when kvm is enabled, or
>>
>> You can use hardware breakpoints as well but it is not required unless
>> the target code can be overwritten (e.g. due to a reset).
>>
>>> "monitor system_reset" after set the breakpoint, but it didn't work for
>> me.
>>> The hardware breakpoint could not been hit anyway.
>>>
>>> I have tried with "-no-kvm", it works normally with breakpoints. But I
>> want
>>> to debug the guest OS with kvm enabled. I don't know whether someone has
>>> met this similar situation.
>>
>> You didn't tell us which version of QEMU (or is it old qemu-kvm?) you
>> are using, what host kernel and which CPU type (AMD vs. Intel). Did you
>> try a recent version of all of them already? I'm currently not aware of
>> gdb problems with QEMU/KVM, I'm rather using it on an almost daily basis
>> (typically git head versions).
>>
> I am using a nested VM.

Oh, "minor" detail ;) - why nested? But this used to work for me with a
patched 3.9+ kernel some while ago.

> My CPU type is intel.
> On L0, the QEMU-KVM version is 1.0, host kernel version: 2.6.32.10,
> kvm-kmod version: 3.2

Try at least the latest kvm-kmod version, but there are even more fixes
in kvm.git. Not sure if any of them has direct impact on your scenario,
but it's generally better to use a recent kernel with this still
experimental feature (VMX nesting).

As this is likely a KVM issue, I'm also CC'ing the corresponding list

Jan

> On L1, the QEMU-KVM version is 1.2, kernel version: 3.2.2, kvm-kmod
> version: 3.2
> On L2, guest kernel version: 2.6.32.10
> I am trying to debug L2 guest kernel on L1 QEMU. It gives me "INT 3"
> related kernel oops.
> I also have tried to debug the L1 guest kernel through L0 QEMU which works
> fine.
> 
>>
>> If you want to debug your issue: there is ftrace to record what KVM
>> events happen, and you can switch gdb into verbose mode as well,
>> comparing the communication between KVM on/off: set debug remote 1.
>>
>> Thanks for your suggestion! I will give that a try.
> 
>> Jan
>>
>>
>>
>

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