Having thread->td_frame point to the most recent frame

[Marcel Moolenaar]

Gang,

I'm revamping kernel debugging support and thread awareness is one
of the features that I'm going to add (at least for remote gdb, but
likely to ddb as well). It's the thread awareness that causes me to
raise the following issue:

When we enter the kernel, we set curthread->td_frame to the current
frame only when we came from user mode. This obviously to make sure
the frame remains valid across nested interrupts/traps.
For debugging you really want the latest (most recent) frame so that
when you switch to a particular thread to produce a backtrace, you
have the right frame (=context). So naturally, you really want to
have thread->td_frame point to the frame that corresponds to the
current kernel entry, whether from userland or not.

One way to deal with this is to link frames so that the frame of the
most recent entry is pointed to by thread->td_frame and the frame of
the previous kernel entry is pointed to by thread->td_frame->tf_prev
(or NULL when there's no previous frame -- or whatever). Obviously,
we need to restore the previous frame when we exit the kernel.

Thus, on entry into the kernel we do something like:
	framep->tf_prev = curthread->td_frame;
	curthread->td_frame = framep;

On exit from the kernel we roll back like:
	framep = curthread->td_frame;
	curthread->td_frame = framep->tf_prev

Q1: Are there any corner cases I'm missing or would such a scheme work?
Q2: What's impacted if and when a new field to struct trapframe is added?
Q3: Are there better alternatives for solving the issue?

-- 
 Marcel Moolenaar	  USPA: A-39004		 marcel at xcllnt.net