Re: kgdb support in vanilla 2.6.2

[Amit S. Kale]

On Thursday 05 Feb 2004 8:41 am, Andi Kleen wrote:
> Andrew Morton <akpm@xxxxxxxx> writes:
> > need to take a look at such things and really convice ourselves that
> > they're worthwhile.  Personally, I'd only be interested in the basic
> > stub.
>
> What I found always extremly ugly in the i386 stub was that it uses
> magic globals to talk to the page fault handler. For the x86-64
> version I replaced that by just using __get/__put_user in the memory
> accesses, which is much cleaner. I would suggest doing that for i386
> too.

May be I am missing something obvious. When debugging a page fault handler if 
kgdb accesses an swapped-out user page doesn't it deadlock when trying to 
hold mm semaphore?

I use the global to let page fault handler that this access is from kgdb so 
don't try to load any pages, report an error instead.

>
> Also what's also ugly in i386 is that it uses ugly hooks in
> traps.c/fault.c. On x86-64 I instead added generic notifiers (see
> include/asm-x86_64/die.h and notify_die in arch/x86_64/kernel/traps.c)
> where both kdb and kgdb and possibly dprobes and other debuggers can hook
> in without conflicting patches for the same files from everybody.
> I would strongly suggest to adopt such a generic framework for i386 too
> to clean up the core kernel <-> debugger interaction. As soon as this
> frame work is in just dropping the stub is is very clean.

I liked these.  Never got time to integrate them.

>
> The x86-64 version should be pretty simple to port to i386 if someone
> is interested ...
>
> Another issue is that for modern gdb and frame pointer less debugging
> with dwarf2 we really need dwarf2 cfi annotation on i386 too. It is
> not as ugly as it used to be because newer binutils have much nicer to
> use .cfi_* mnemonics to generate the dwarf2 unwind table. x86-64 uses
> that now thanks to Jim Houston. It only works with uptodate binutils,
> but I guess that's a reasonable requirement. It's a bit intrusive in
> entry.S, but not too bad compared to the old way (take a look at
> arch/i386/kernel/vsyscall-sysenter.S to see how the old way looks
> like) Having the dwarf2 unwind information in the kernel vmlinux is
> useful even independent of kgdb for other tools that look at crash
> dumps.

George has coded cfi directives i386 too. He can use them to backtrace past 
irqs stack.
-- 
Amit Kale
EmSysSoft (http://www.emsyssoft.com)
KGDB: Linux Kernel Source Level Debugger (http://kgdb.sourceforge.net)

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