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I'm debugging a kernel and i want to know when the cr3 register is changed. I know how to set a watchpoint on a general purpose register like eax and others.
The problem is, since gdb does not have access to control registers, setting a watchpoint on cr3 does not work.
So, is it possible to set a watchpoint from the qemu monitor? If yes, how?
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To enable connection to the GDB server, you need to pass in a parameter to QEMU that specify the hostname and port it should listen on. Makes QEMU's GDB server listen on host hostname on port port. Generally the hostname is "localhost" and the port can be anything, as long as you can connect to it.
Set a watchpoint that will break when watch expr is read by the program. Set a watchpoint that will break when expr is either read or written into by the program. This command prints a list of watchpoints, breakpoints, and catchpoints; it is the same as info break . GDB sets a hardware watchpoint if possible.
Setup a debugger connection to a Windows Image on QEMUDownload and install QEMU on Windows. Configure a target QEMU Virtual Windows Image to launch with the required network and BIOS/UEFI settings for debugging. Start the QEMU environment, using the configured launch script. Start the gdbserver on QEMU.
Hardware watchpoints - allow execution to halt when a read or write access is made to a data variable address. Count Event - can be used to measure clock cycles between two points in the code. Data Access Count - can be used to determine the number of times a data variable address has been accessed.
Sorry, there's 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'll see that it doesn't do anything that would notify anybody about CR3 updates, it just puts the new value into the internal CPU state structure.)
The best you can do for this sort of thing is to run with two debuggers (one connected to QEMU's gdbstub to talk to the guest, and one directly debugging QEMU itself). Then you can put a breakpoint on cpu_x86_update_cr3() in QEMU and see what's going on then. You need to know a fair amount about QEMU's internals to be able to do this effectively, though...
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