PowerMac G5 hangs/crashes on boot: 10.2, 11.0-RCx

[Mark Millard]

On 2016-Sep-12, at 2:00 PM, Nathan Whitehorn <nwhitehorn at freebsd.org> wrote:

> On 09/11/16 12:04, Krzysztof Parzyszek wrote:
>> On 9/10/2016 7:12 PM, Mark Millard wrote:
>>> 
>>> Your report that you have some sort of failure after booting during the likes of buildkernel is new as far as I know. Sending the list (and/or a bugzilla report) about the failure mode and related information may be appropriate and help in the future. (May be I missed an already existing report.)
>> 
>> I think the issues with hangs are related to the temperature sensors.  I was getting console messages that the media temp sensor was reporting 120C! I realized that as I replaced the hard drive, I changed the position of the drive in a way that exposed the sensor to the wind from the fans (it used to be angled a bit). Following this line of thought, I stuck a piece of paper in front of the sensor and things seem to work. At least work better than before. I have ordered a replacement sensor from eBay, so hopefully that problem will go away.
>> 
>> Regarding your "hack": it works perfectly! So far, my system (10.3 at the moment) has booted every single time, no exceptions, traps, unexpected conditions. This is too good to remain as a "hack". I have no experience debugging the kernel and I don't know ABI the OFW follows, but I wouldn't mind digging a bit deeper into it, if someone shared some pointers.
> 
> I've finally understood why this helps. Open Firmware runs in virtual mode on the Powermac G5. This runs inside the kernel page table, which preserves all address translations made by OF before the kernel starts; as a result, the kernel address space is a strict superset of OF's.
> 
> Where this explodes is if OF uses an unmapped SLB entry. The SLB fault handler runs in real mode and refers to the PCPU pointer in SPRG0, which blows up the kernel. Having a value of SPRG0 that works for the kernel is less fatal than preserving OF's value in this case.

Actual theory of operation material! Great! I've no clue how I would have figured out the Apple Open Firmware side of the context so I'd still be stuck with just observation of the overall change in behavior.

Presuming that you are correct: no longer "just a hack". That is great news.

It does sound like the Open Firmware side gets some risk of picking up the wrong address map with the change. So avoiding Open Firmware in the kernel via the usefdt=1 sounds like it would still be appropriate.

> I believe that OF's SPRG0 is maintained only for compatibility with some G4 Apple hardware, the eMac in particular, but will check and we can move on with this. I think it should be safe to wrap this in an #ifdef __powerpc64__.
> -Nathan

Again I've no clue how I would figure such a thing out. But, if some powerpc Macs are the only reason GENERIC preserves SPRG0 and preserving works for the other powerpcs so that detecting PowerMac vs. not is not really required to decide on the SPRG0 handling (at least for powerpc64), that too is great news.


Thanks for continuing to look into why the SPRG0 change might have a solid justification!

>> 
>> This is all with powerpc64.
>> 
>> I'll try checking if it helps with 11.0 next.
>> 
>> -Krzysztof
>> 
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===
Mark Millard
markmi at dsl-only.net