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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