I don't think GDB can help us very much on that, as it's only working with virtual addresses. You can't set a physical address breakpoint because the protocol doesn't allow it. So I guess it can be only done in the monitor.
So, for you that you know very well the QEMU code, and suppose that I want to do a quick modification to stop on an address write and print the backtrace. How I could do it?? I want a one line modification to the code (with the address hardcoded) to stop the monitor. That should be easy, right? Many thanks! El 26 de agosto de 2011 20:12, Jan Kiszka <jan.kis...@siemens.com> escribió: > On 2011-08-26 20:02, Blue Swirl wrote: > > 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? > > When debugging tiny SMP races, yes as it may change the timing. > > But MMIO based watchpoints may indeed work as well and also would > overcome the "Who needs more than 4 hw breakpoints?". But configuring > watchpoints on physical address or manually switching between soft and > hard ones need some configuration interface, ideally one that gdb can > drive. We also need some interface to finally add monitor trap support > (single stepping without injecting TF into guest flags, an Intel VT-x > feature). Just takes someone to do all this... :) > > Jan > > -- > Siemens AG, Corporate Technology, CT T DE IT 1 > Corporate Competence Center Embedded Linux > -- Zeus Gómez Marmolejo Zet - The x86 (IA-32) open implementation http://zet.aluzina.org
