Re: [Qemu-devel] Failed to use gdb with qemu 15.1 (with and without kvm support)

[Francis Moreau]

On Sun, Jan 22, 2012 at 8:28 PM, Jan Kiszka <address@hidden> wrote:
> On 2012-01-22 20:25, Francis Moreau wrote:
>> Hello,
>>
>> Thanks for you hints !
>>
>> On Sun, Jan 22, 2012 at 7:49 PM, Jan Kiszka <address@hidden> wrote:
>>> On 2012-01-22 15:20, Francis Moreau wrote:
>>>> Hello,
>>>>
>>>> I'm trying to debug a 64 bits kernel (3.2) using qemu (with and
>>>> without KVM support). The host is also 64 bits. Version of gdb I'm
>>>> using is 7.1.
>>>>
>>>> To do that I simply start qemu like the following:
>>>>
>>>>  $ qemu-system-x86_64 -kernel bzImage -append "root=/dev/sda1
>>>> console=ttyS0 3" -serial stdio -S -s
>>>>
>>>> and in another term, I start gdb like this:
>>>>
>>>>  $ gdb vmlinux
>>>>  $ target remote locahost:1234
>>>>  $ b prepare_namespace
>>>>  $ continue
>>>>  Continuing.
>>>>  Remote 'g' packet reply is too long: af75919effffffff0.............
>>>>
>>>> So it fails when gdb stops on the breakpoint with the above message.
>>>>
>>>> If I try to dump the backtrace I got:
>>>>
>>>>  $ bt
>>>>  Target is executing.
>>>>  $ info thread
>>>>  * 1 Thread 1 (CPU#0 [running])  (running)
>>>>
>>>> But the VM seems to be stopped because if I'm asking the status to qemu:
>>>>
>>>>  $ info status
>>>>  VM status: paused
>>>>
>>>> I also tried qemu with KVM support but I get one more problem: gdb is
>>>> ignoring my breakpoint.
>>>>
>>>> Could anybody help me to make gdb work ?
>>>
>>> When stopping the guest with -S before it booted, gdb will interrupt it
>>> while it is still in 16-bit real mode. Later on, when Linux runs, the
>>> guest is in 64-bit protected mode. gdb is not prepared for such a
>>> switch. All you can do:
>>>
>>> - let the guest run until it surely reached 64-bit mode
>>> - interrupt it and set a breakpoint at the desired early-boot location,
>>
>> So I let the kernel boot, and then I'm trying to start and connect gdb
>> to qemu but unfortunately gdb is segfaulting when trying to connect :(
>
> Try gdb 7.3 or even latest development version (the latter is required
> for module debugging - just in case).

OMG it's working !

One weird thing though: if I put "target remote localhost:1234" in
.gdbinit then I'm getting this in gdb:

   /home/fmoreau/.gdbinit:1: Error in sourced command file:
   Remote 'g' packet reply is too long: c0f6ba3d0088fff....

I've no problem if I don't use .gdbinit.

That's sad that setting arch doesn't work.

>
>>
>>>  important: if using KVM, set a hardware breakpoint!
>>
>> ah ok good to know, I'll try to use hw breakpoints.
>
> The reason is that software breakpoints are implemented under kvm by
> patching breakpoint instructions into the guest - and those get
> overwritten when reloading the kernel after reboot.

Thanks for the information, using hw breakpoint with kvm works !

Thanks a lot !
-- 
Francis