[Bug 205458] 11.0-CURRENT/10-STABLE powerpc64: a PowerMac G5 specific sys/powerpc/ofw/ofw_machdep.c change for reliable PowerMac G5 booting (with lots of RAM)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=205458

--- Comment #17 from commit-hook at freebsd.org ---
A commit references this bug:

Author: jhibbits
Date: Tue Mar  7 22:11:58 UTC 2017
New revision: 314885
URL: https://svnweb.freebsd.org/changeset/base/314885

Log:
  Fix booting with >4GB RAM on PowerMac G5 hardware

  ===
  From Nathan Whitehorn:

  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.

  ===

  The result of this is seemingly random panics from NULL dereferences, or
hangs
  immediately upon boot.  By not restoring SPRG0 for Open Firmware entry the
  kernel PCPU pointer is preserved and SLB faults are successful, resulting in
a
  stable kernel.

  PR:           205458
  Reported by:  several (over bugzilla, lists, IRC)
  Reviewed by:  andreast
  Tested by:    many (various forms)
  MFC after:    2 weeks

Changes:
  head/sys/powerpc/ofw/ofw_machdep.c

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