Re: [parisc-linux] 64-bit kernel crashes on my c3600

[Randolph Chung <randolph@tausq.org>]

> One of the problems with this crash is that enabling EARLY_CONSOLE
> doesn't help.  The exact same configuration boots fine in 32-bit mode.
> I'm building from the same tree (with O=) so there's no question of patch
> skew.  Turning on DISCONTIGMEM does not help.  The HPMC points inside
> the code generated by the save_general macro just past skip_save_ior
> inside the intr_save function in entry.S

i've found out some more info about this problem, but still no clue why
it's happening....

at the end of head.S, when we branch to virtual space, the first virtual
insn access (to start_kernel) causes a itlb miss fault (as expected).
For some reason, the itlb handler is not able to find the page for
start_kernel in the page table, so it attempts to call the fault handler
(handle_interruption, via intr_save). However, in intr_save, as soon as
we switch to virtual space (virt_map, right before the save_general
macro call), we immediately cause another itlb miss fault, which fails,
and calls intr_save again. Each time intr_save is called, we create a
new stack frame. Eventually, the stack pointer points past valid phys
addr space, and the machine HPMCs.

The question is, why does the itlb miss handler fail to find the mapping
for start_kernel? On my kernel, start_kernel is at 0x1056xxxx, which is
well within the 16MB initially mapped in head.S. I went through the code
in head.S several times and it seems to be correct. I also don't quite
understand how this part of the code, which is all in assembly, can
behave differently between gcc-3.3 and gcc-3.4. I tried to move the
initial-VM initialization code in head.S much closer to the rfi (with
the hypothesis that some intervening code had trashed the page table)
but that doesn't seem to help. I also had a theory that perhaps the
different gcc versions were expanding the #define's differently for
offsets.h, but that doesn't seem to be the case either... so i'm out of
ideas :( 

if it helps, what i see is that in L2_ptep, 
    ldw,s \index(\pmd),\pmd
is returning with \pmd == 0

weird....

randolph
-- 
Randolph Chung
Debian GNU/Linux Developer, hppa/ia64 ports
http://www.tausq.org/
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