Re: [Qemu-devel] Failed to set a breakpoint on start_kernel

[Jan Kiszka]

On 2012-03-17 12:25, Laurent Vivier wrote:
> Le samedi 17 mars 2012 à 09:53 +0100, Jan Kiszka a écrit :
>> On 2012-03-16 03:43, Wei Yang wrote:
>>> All
>>>
>>> I like qemu very much and know it could debug the kernel.
>>>
>>> I tried what I searched on web but couldn't stop at the break point.
>>> Below is what I did.
>>>
>>> 1. Both host and guest installed the same OS, Fedora16 x86_64.
>>>
>>> 2. Compile the qemu with
>>>     ./configure --target-list=x86_64-softmmu --enable-kvm
>>> --enable-debug-tcg --enable-debug --enable-trace-backend=simple
>>>
>>> 3. With this command I can boot up my guest.
>>>     ./../qemu/x86_64-softmmu/qemu-system-x86_64 -enable-kvm -smp 4 -m
>>> 1024  -boot dc fedora16.img -monitor stdio
>>>
>>> 4. I git clone the kernel source in the guest and make a new kernel and 
>>> initrd.
>>>     I start the guest with this new kernel successfully
>>>
>>> 5. I copy out the initrd.img and the .config of kernel to host.
>>> compile the kernel on host.
>>>     the kernel source code is identical on host and gueset,
>>>
>>> 6. I start the guest with the kernel and initrd on host
>>>     ./../qemu/x86_64-softmmu/qemu-system-x86_64 -enable-kvm -smp 4 -m
>>> 1024  -boot dc fedora16.img -monitor stdio -kernel
>>> ~/git/linux-yinghai/arch/x86_64/boot/bzImage -initrd
>>> ~/git/debug/initramfs-3.0.0.img -append
>>> "root=/dev/mapper/vg_wizard-lv_root ro rd.lvm.lv=vg_wizard/lv_root
>>> rd.md=0 rd.lvm.lv=vg_wizard/lv_swap"
>>>
>>>     This works fine.
>>>
>>> 7. Then I start the guest with gdbstub option
>>>     ./../qemu/x86_64-softmmu/qemu-system-x86_64 -enable-kvm -smp 4 -m
>>> 1024  -boot dc fedora16.img -monitor stdio -kernel
>>> /home/ywywyang/git/linux-yinghai/arch/x86_64/boot/bzImage -initrd
>>> /home/ywywyang/git/debug/initramfs-3.0.0.img -append
>>> "root=/dev/mapper/vg_wizard-lv_root ro rd.lvm.lv=vg_wizard/lv_root
>>> rd.md=0 rd.lvm.lv=vg_wizard/lv_swap" -S -gdb tcp::4321
>>>
>>>     Then the guest stop at the beginning.
>>>
>>> 8. Attach the gdb in the kernel source directory
>>>     gdb
>>>     file vmlinux
>>>     target remote localhost:4321
>>>     b start_kernel
>>>     c
>>>
>>>    Then the guest will run very happily....
>>>
>>>     Also use the "info b " could show the break point is set.
>>>
>>> Which step I made a mistake?
>>
>> Two major issues with this procedure:
>>
>> 1. When using kvm, a soft breakpoint (as set by 'b') will inject a trap
>> instruction into the guest image - which is not yet loaded after the
>> bios ran. You need to use a hardware breakpoint in this case.
>>
>> 2. Due to gdb limitations, you cannot switch between 16/32-bit mode (the
>> CPU starts in 16 bit) and the 64-bit mode of kernel within the same gdb
>> session. Therefore:
>>  - let the target run into Linux is active
>>  - attach gdb
>>  - issue "hw start_kernel"
>>  - reboot (e.g. "monitor system_reset")
>>  - you will hit the breakpoint, and gdb will be usable
> 
> You can also try my patch :
> 
> http://patchwork.ozlabs.org/patch/137543/

Unless there is a use case beyond this x86 band-aid, lets focus on
getting gdb right. Reminds me that gdb folks asked me to file a bug
about this - which I still need to do. :-/

Jan

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