Re: [Qemu-devel] [PATCH] x86: Fix x86_64 'g' packet response to gdb from 32-bit mode.

[Pedro Alves]

On 11/04/2016 04:34 PM, Peter Maydell wrote:
> On 4 November 2016 at 16:29, Richard Henderson <address@hidden> wrote:
>> On 11/03/2016 05:35 PM, Doug Evans wrote:
>>>
>>> The remote protocol can't handle flipping back and forth
>>> between 32-bit and 64-bit regs. To compensate, pretend "as if"
>>> on 64-bit cpu when in 32-bit mode.
>>>
>>> Signed-off-by: Doug Evans <address@hidden>
>>> ---
>>>  target-i386/gdbstub.c | 52
>>> ++++++++++++++++++++++++++++++++++++++-------------
>>>  1 file changed, 39 insertions(+), 13 deletions(-)
>>
>>
>> Reviewed-by: Richard Henderson <address@hidden>
> 
> Has anybody ever looked at fixing this on the gdb end?
> (There are some similar cases for ARM CPUs, and actually
> being able to debug across 32/64 boundaries, big/little
> endian boundaries etc would be quite handy in some
> situations.)
> 

A while ago I was chatting with Andy Lutomirski about this,
and he was saying that at least for x86_64, he'd prefer if gdb
always thought of the register file as 64-bit, even if
in 16-bit/32-bit mode.

 
http://lists-archives.com/linux-kernel/28605147-x86-signal-rewire-the-restart_block-syscall-to-have-a-constant-nr.html
 
http://lists-archives.com/linux-kernel/28605329-x86-signal-rewire-the-restart_block-syscall-to-have-a-constant-nr.html

I never followed up with that, which is totally lame of me.

Even if we did that, when debugging 32-bit mode code,
I think you'd want gdb to present a 32-bit view of the program,
so that printing expressions uses the correct types,
calling 32-bit/64-bit functions in the inferior builds
a call frame using the right ABI, etc. (and that exposes
other challenges like maybe needing to switch modes
while the infcall is running if you call a function of
the "wrong" mode, etc.)  I believe the MIPS 64-bit gdb port actually
works that way - it always transfers 64-bit registers across the
wire, and then gdb presents 32-bit registers to debugging an elf
of a 32-bit ABI.  I think it's the only port that does that.
Whatever approach is taken, I suspect that there's a good amount
of work needed to make things work completely seamlessly, though.

Thanks,
Pedro Alves