Debugging with gdbserver and qemu, how to set a watchpoint in the control register, cr3

Question

I am debugging the kernel and I want to know when the cr3 register changes. I know how to set a watchpoint in a general register, for example, eax and others.

The problem is that since gdb does not have access to control registers, setting the watchpoint on cr3 does not work.

So, is it possible to set a breakpoint from the qemu monitor? If so, how?

Answer 1

Sorry, there is no way to do this from the QEMU monitor. (If you look at target-i386 / helper.c: cpu_x86_update_cr3 () in the QEMU sources, you will see that it does nothing to notify anyone about CR3 updates, it just adds a new value to the internal state of the CPU structure .)

The best thing you can do for this kind of thing is to run with two debuggers (one of which is connected to QEMU gdbstub to talk with the guest, and also directly debug QEMU). You can then set a breakpoint on cpu_x86_update_cr3 () in QEMU and see what happens next. You must be aware of the number of QEMU internal organs in order to be able to do this efficiently, though ...