Hi Linus,
that one boots OK now, thanks (applied on top of mine but that should
not matter). I'll run a test on the actual hardware once the previous
one is done.
Cheers,
Michael
Am 19.06.2021 um 14:13 schrieb Linus Torvalds:
On Fri, Jun 18, 2021 at 6:54 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
The registers being zero at that point is actually expected, so that's
not much of a hint. But yeah, clearly I got some stack initialization
offset or something wrong there, and I don't know modern m68k nearly
well enough to even guess where I screwed up.
Oh. I think I might have an idea.
In that ret_from_kernel_thread code, after it returns from thge kernel
thread, I did
addql #4,%sp
RESTORE_SWITCH_STACK
jra ret_from_exception
where that "RESTORE_SWITCH_STACK" loads the user space registers from
the user-space switch stack.
BUT.
The switch stack also has that "retpc", and that one should just be popped.
So I think that code should read
addql #4,%sp
RESTORE_SWITCH_STACK
addql #4,%sp
jra ret_from_exception
because we want to restore the switch stack registers (they'll all be
0) but we then want to get rid of retpc too before we jump to
ret_from_exception, which will do the RESTORE_ALL.
(The first 'addql' is to remove the argument we've pushed on the stack
earlier in ret_from_kernel_thread, the second addql is to remove
retpc).
All the *normal* uses of RESTORE_SWITCH_STACK will just do "rts", but
that's because they were called from the shallower stack. The
ret_from_kernel_thread case is kind of special, because it had that
stack frame layout built up manually, rather than have a call to it.
In that sense, it's a bit more like the 'do_trace_entry/exit' code,
which builds that switch stack using
subql #4,%sp
SAVE_SWITCH_STACK
and then undoes it using that same
RESTORE_SWITCH_STACK
addql #4,%sp
sequence that I _should_ have done in ret_from_kernel_thread. (Also,
see ret_from_signal)
I've attached an updated patch just in case my incoherent ramblings
above didn't explain what I meant. It's the same as the previous
patch, it just has that one extra stack update there.
Does _this_ one perhaps work?
Linus
