How to debug QEMU live migration with GDB without causing the destination VM to hang?

Question

I'm trying to debug the QEMU source code to track dirty page updates during live migration. My goal is to inspect the behavior inside functions like cpu_physical_memory_set_dirty_lebitmap() on the destination server, which handles syncing of KVM dirty pages.

When I simply use QEMU logs (printf) inside that function, it only prints 0 dirty pages — until I connect to the destination VM using RealVNC, then it starts printing proper dirty page counts. This suggests some laziness or delay in page updates that I want to analyze deeper.

To be clear: I have run a memory-intensive process inside the guest VM to intentionally dirty pages, so dirty pages should be getting tracked.

So I turned to GDB, but I'm running into issues.

1. Built QEMU from source with debugging enabled using --enable-debug.

2. Launched the destination VM with:

   qemu-system-x86_64 ... -s -S
   

   • -s opens a GDB server on port 1234.
   • -S tells QEMU to not start the CPU until GDB gives the continue command.

3. Attached GDB like this:

   gdb qemu-system-x86_64
   (gdb) target remote localhost:1234
   (gdb) break cpu_physical_memory_set_dirty_lebitmap
   (gdb) continue
   

But the destination VM (and the source as well, which is normal in post-copy live migration) hangs and displays an unresponsive GUI. Nothing progresses from there.

4. If I remove -S, the migration works fine and the destination VM doesn’t hang — but I can’t insert breakpoints early enough, so I lose the ability to properly debug.

5. In the QEMU documentation, it suggests launching GDB like this:

   gdb vmlinux
   

   But from what I understand, that's meant for debugging the Linux kernel, not QEMU itself. Since I'm trying to debug the QEMU source code, I believe the correct approach is:

   gdb qemu-system-x86_64
   

Question

How can I debug QEMU with GDB during live migration, without the destination VM or the migration process hanging when I use -S?