Re: [Qemu-devel] Physical address debugging

[Blue Swirl]

2011/8/26 Jan Kiszka <address@hidden>:
> On 2011-08-26 19:34, Blue Swirl wrote:
>> 2011/8/26 Jan Kiszka <address@hidden>:
>>> 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?