Re: [ANNOUNCE] Merkey's Kernel Debugger

[jmerkey@wolfmountaingroup.com]

I found a problem with APIC NMI support which seems to affect all the
debuggers, but appears machine specific -- at least I can reproduce it
with all of the modules MDB, KDB, and KGDB modules on my ACER 2410 dual
core laptop.  It explains the mysterious hangs I would see in KDB all the
time on SMP systems.

The call:

send_IPI_allbutself(vector)

will hard hang an on ACER laptop with dual core processors if issued while
any one of the processors are actively inside an INT 1 handler, then take
a SECOND NMI inside of this path, and nest.   It hangs the requesting
(focus) processor during nested interrupts if a target processor is A)
inside an INT 1 exception B) takes an NMI interrupt C) returns from the
NMI back into the INT1 D) receives a second NMI.

I am aware that a second NMI will not propagate to a processor currently
servicing an NMI until the processor sees an IRET instruction (at least
this is how intel worked years back).

I have not been able to reproduce it on the Xeon based motherboards.  I
have seen the APIC bus hang this way on my other OS project -- when the
APIC was programmed incorrectly, and assume it must be a bug in the APIC,
how the APIC is programmed by Linux, etc.

I am coding around the problem to prevent such convoluted nesting levels
in MDB (this was from testing) but this was the final test for enabling
SSB and all the fixes before I post and rc3 patch series which really
cleanup up the code, and there's a mystery with send_IPI_allbutself().

Jeff