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

From: Jamie Lokier
Subject: Re: [Qemu-devel] Re: [PATCH 4/4] gdbstub: x86: Switch 64/32 bit registers dynamically
Date: Mon, 29 Jun 2009 23:00:45 +0100
User-agent: Mutt/1.5.13 (2006-08-11)

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).

-- Jamie

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