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

[Jan Kiszka]

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

Jan

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