Re: Ideas to Improve GDB Stub in Qemu for i8086

[Davidson Francis]

On Thu, Dec 19, 2024 at 09:51:13AM -0700, Warner Losh wrote:
> I might be interested in debugging some i8086 stuff for my Venix/86
> emulation
> project.
> 
> I'm curious, though. I see special cases just for EIP being returned as (CS
> << 4):EIP
> and similar for ESP being just (SS << 4):SP. What does the debugger do,
> though,
> for other cases where you need segment:offset addresses? Does it translate
> properly
> behind the scenes in ways it doesn't do for ESP/EIP or do you have to
> examine those
> addresses by hand as well? If I'm chasing a linked list with 16-bit
> addresses that assume
> a particular DS, how does that work?
>

You've brought up a very interesting point. Indeed, in the case of 
linked lists and similar structures, there’s no way for GDB or QEMU to 
reliably determine which segment an address might belong to—perhaps 
only guesses, which are far from ideal.  

That said, the approach I'm proposing doesn’t solve all problems and 
still requires users to handle these conversions manually or through GDB 
scripts.  

The main goal of my idea is simply to reduce the initial effort required 
to debug i8086 code, avoiding workarounds with architecture XML files, 
and proper instruction disassembly and stack dumps out-of-the-box.

Kind regards,
Davidson Francis.