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

[Jan Kiszka]

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,
  important: if using KVM, set a hardware breakpoint!
- continue and reboot the guest without detaching gdb
- on next boot, the breakpoint will trigger

HTH,
Jan

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