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

[Laurent Vivier]

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/

Laurent