Re: [Qemu-devel] Re: [PATCH 4/4] gdbstub: x86: Switch 64/32 bit registers dynamically

[Jan Kiszka]

Jamie Lokier wrote:
> Jan Kiszka wrote:
>> Daniel Jacobowitz wrote:
>>> On Mon, Jun 29, 2009 at 04:53:45PM +0200, Jan Kiszka wrote:
>>>> Just to recall the situation (again, please actually try it): if you
>>>> have to debug code that switches between 16/32 bit and 64 bit, you
>>>> _can't_ debug the 16 or 32 bit part as gdb will stumble and fall over
>>>> qemu sending 64-bit register layout for 16/32 bit code. That is a gdb
>>>> limitation, but this patch is about dealing with it until it's resolved
>>>> in gdb.
>>> Remind me why you can't just tell GDB that the target is 64-bit
>>> despite whatever file you've given it?
>> Because gdb mixes up arch capability and current operation mode on x86.
>> It always tries to disassemble according to the set arch. Moreover, it
>> will misinterpret the registers as being valid across all 64 bits, not
>> just 16 or 32. I haven't looked into further side effects, but I bet
>> there are more.
> 
> On a 64-bit CPU in 16/32-bit mode, all 64 register bits _are_ valid
> aren't they?  (But not useful, as far as I know.  Unless there's a
> 64-bit equivalent to i386's "big real" modes and such).

I meant they are invalid in the sense that, e.g., 32 bit code will not
modify the bits 32..63, thus r*x may not always equal e*x.

But even more problematic is the fact that frame unwinding does not work
if gdb applies 64 bit mode while the target is doing something
completely different. A "set arch i386:x86-64" workaround for this
problem simply does not work.

Jan

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