Re: KGDB 2.0.3 with fixes and development in ethernet interface

[Amit S. Kale]

On Thursday 22 Jan 2004 4:30 am, George Anzinger wrote:
> Amit S. Kale wrote:
> > On Sunday 18 Jan 2004 1:24 am, George Anzinger wrote:
> >>Amit S. Kale wrote:
> >>>On Saturday 17 Jan 2004 6:53 am, George Anzinger wrote:
> >>>>>This seems to be needed (if I unselect CONFIG_KGDB_THREAD, I get
> >>>>>compile error on x86-64).
> >>>>
> >>>>Amit, could you explain why this is an option?  It seems very useful
> >>>> and not really too much code...
> >>>
> >>>It saves all registers before switch_to. It does that two times at
> >>>present. Once (implicit) register save by gcc and an explicit register
> >>>save in arch/<proc>/kernel/entry.S Second register save in kern_schedule
> >>>generates a pt_regs structure which gdb can get all registers at once
> >>>from. If it's omited, gdb has to figure out where gcc has saved
> >>> registers from the non-standards code in switch_to, which it can't do
> >>> correctly all the time.
> >>>
> >>>We can have a check for (a new variable) debugger_enabled before calling
> >>>kern_schedule. That'll be add negligible overhead and will allow extra
> >>>thread info to be saved only when a debugger is enabled. There will not
> >>>be any need for CONFIG_KGDB_THREAD also.
> >>
> >>I don't seem to have such a problem with the mm kgdb.  No kern_schedule
> >>there...
> >
> > Have you confirmed that you see correct values for all the registers? I
> > had found some problems with gdb not being able to locate all the
> > registers correctly. Explanation on the problem below:
> >
> > Besides getting gdb's fault there  is one more benefit of kern_schedule.
> > All threads are shown by gdb as sleeping _at_ the place where schedule
> > call is present: So instead of gdb showing all threads sleeping in
> > __switch_to, it shows several functions. That's better than having to
> > look at back trace of each thread to figure out what it is.
>
> The mm version of kgdb does this, but by lying to gdb during the info
> thread command.  IMNSHO, the real way to do this is to process the info
> thread output with an awk script, but I haven't done that just yet.  The
> problem with lying to gdb is that it sometimes remembers...
>
> > This functionality is complemented by printing of thread names in 2.0.X
> > kgdb info threads listing.
>
> as in the mm version.
>
> > Now back to gdb problem of not being able to locate registers.
> > schedule results in code of this form:
> >
> > schedule:
> > framesetup
> > registers save
> > ...
> > ...
> > save registers
> > change esp
> > call switchto
> > restore registers
> > ...
>
> I have not analyzed this as yet.  However, it does seem to me to be the
> same problem as trying to bt through an interrupt frame.  The correct way
> to do this is to build the dwarf frame descriptors.  I have done this for
> the interrupt frame and intend to send said patch out in a day or so.

Great! I had to do it this ackward way:

i386 ->

 	ALIGN
 common_interrupt:
 	SAVE_ALL
+	movl %esp, %eax
+/* Create a fake function call followed by a fake function prologue to fool
+ * gdb into believing that this is a normal function call. */
+	pushl EIP(%eax)
+
+common_interrupt_1:
+	pushl %ebp
+	movl %esp, %ebp
+	pushl %eax
 	call do_IRQ
+	addl $12, %esp
 	jmp ret_from_intr
 
Powerpc ->
 	int irq, first = 1;
+	unsigned long *lrptr;
+	if (!(regs->msr & MSR_PR)) {
+		/* Came in from the kernel. Save call link. */
+		lrptr = (unsigned long *)(regs->gpr[1] + 4);
+		*lrptr = regs->nip;
+	}
   
I guess your patch will fix this problem for i386 only. Any ideas on doing it 
for powerpc too?


>
> Could be I should look at the above and do the right thing for it also.
>
> > ...
> >
> > GDB can't analyze code other than frame setup and registers save. It may
> > not show values of variables that are present in registers correctly.
> > This used to be a problem some time ago (gdb 5.X). Perhaps gdb 6.x does a
> > better job. hmm...
> > May be its time I should look at gdb's x86 register info code again.
>
> The latter gdbs (not sure when) are using dwarf frame code.  The asm code
> needs to be anotated to make this work.  There is a problem I reported a
> couple of days ago with the dwarf expression code.  I think it is fixed,
> but that would still be the cvs version of gdb.
>
> I think I would rather NOT modify kernel code and put up with the register
> loss in some cases.  I am MUCH more concerned with unexpected behavior due
> to the code changes.  Heisenberg is NOT our friend.

Agreed. I am not happy with the CONFIG_KGDB_THREAD code in entry.S That's the 
reason I have kept it optional.

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

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