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

[Francis Moreau]

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 :(

>  important: if using KVM, set a hardware breakpoint!

ah ok good to know, I'll try to use hw breakpoints.

Thanks
-- 
Francis