2011/8/26 Jan Kiszka <jan.kis...@siemens.com>: > On 2011-08-26 19:34, Blue Swirl wrote: >> 2011/8/26 Jan Kiszka <jan.kis...@siemens.com>: >>> On 2011-08-26 16:21, Zeus Gómez Marmolejo wrote: >>>> Hi all, >>>> >>>> I just want to do a very simple task. I'm using QEMU for debugging a new OS >>>> and I see that there is a memory region that is being updated by an unknown >>>> task. I don't know if it's another process or the kernel itself. >>>> >>>> So I would like to put a physical memory watchpoint. I don't know how to do >>>> that. As the GDB stub is operating using virtual addressing, I cannot place >>>> a watchpoint on an address as it's not stopping. >>>> >>>> To make it short, I want the equivalent of the Bochs command "watch write >>>> addr", which inserts a write watch point at physical address 'addr'. I >>>> haven't seen this is possible on the QEMU monitor... >>> >>> Indeed, it isn't. It's impossible with KVM (due to lacking hardware >>> support) but would be feasible with TCG if you extend (or ad-hoc hack) >>> QEMU code accordingly. >> >> Why impossible? The existing watchpoints replace original physical >> memory with io_mem_watch MMIO, virtual addresses are translated to >> physical in exec.c:tlb_set_page(). The only missing piece is to allow >> setting watchpoints with physical addresses directly. >> >> Since KVM can handle MMIO, I think it should also be able to handle >> watchpoints. > > KVM uses x86 debug registers for watchpoints. And the SDM states: > > 16.2.1 Debug Address Registers (DR0-DR3) > > Each of the debug-address registers (DR0 through DR3) holds the 32-bit > linear address of a breakpoint (see Figure 16-1). Breakpoint comparisons > are made before physical address translation occurs. [...]
KVM could be changed to use the TCG version. What's the benefit of using debug registers, speed? Is that important for debugging?
