Re: [RFC] Adding %cs and %ss for AMD64 to GDB

["Amit S. Kale" <amitkale at emsyssoft dot com>]

Mark,

Linux kgdb indeed will not have much of a problem. I anyway have posted a gdb 
with kernel module loading feature at kgdb.sourceforge.net. It's going to 
stay there for till the time this feature makes it to official gdb. I can add 
this change also to that gdb.

I guess it's time to introduce a version stamp on remote protocol. That way 
any further modifications to the protocol will not result in surprises from 
gdb-gdbserver mismatches.

On Thursday 01 Jan 2004 6:21 am, Mark Kettenis wrote:
> Folks,
>
> GDB doesn't handle %cs and %ss for AMD64 yet.  I'm not sure why they
> were omitted (might be because they're not particularly useful in long
> mode), but there are cases where we might need them.  I might add
> these registers after the current last register (%mxcsr), but it just
> seems so much more logical to have them close to the other segment
> registers.  However, since GDB's register numbering influences the
> remote interface, I cannot simply do this.
>
> So here's my question: How bad would it be to change the remote
> protocol for AMD64?
>
> I'm proposing to add %cs and %ss just after %eflags.  This would mean
> that the segment registers, the floating-point registers and the SSE
> registers will shift.  Using a new GDB with an old gdbserver, or an
> old remote stub, will mean those registers will contain bogus values.
>
> I did a small survey of the Open Source projects that might make use
> of GDB's remote interface to see what the impact would be:
>
> * Linux kgdb doesn't use anything beyond %eflags yet.  Impact would be
>   zero.
>
> * FreeBSD kgdb supplies %cs and %ss in the slots that are now reserved
>   for %ds and %es.  It provides zero for %fs and %gs (which it thinks
>   are %ds and %es).  The impact would be positive!
>
> * NetBSD kgdb doesn't seem to support remote debugging for AMD64 yet.
>
> * Gdbserver supplies %cs and %ss in the slots that are now reserved
>   for %ds and %es, %ds and %es in the slots for %fs and %gs, and %fs
>   and %gs in the slots for %st0 and %st1.  So the situation for the
>   segment registers would actually improve!  However, for the
>   floating-point and SSE registers the situation would become worse,
>   although right now the most important floating-point registers %st0
>   and %st1 might be unreliable.
>
> Note that the problems with gdbserver can be solved by simply
> upgrading gdbserver on the remote machine, which in most cases won't
> bee too difficult.  I can't imagine that there are many embedded AMD64
> systems out there yet.
>
> From the above I conclude that it wouldn't be too bad to add the %cs
> and %ss registers the way I propose.  However, don't hesitate to tell
> me if you think differently.
>
> Mark

-- 
Amit Kale
EmSysSoft (http://www.emsyssoft.com)
KGDB: Linux Kernel Source Level Debugger (http://kgdb.sourceforge.net)