[Bug 234539] [PowerPC64] panic: FPU already enabled for thread

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234539

--- Comment #5 from commit-hook at freebsd.org ---
A commit references this bug:

Author: luporl
Date: Thu Feb 14 15:15:32 UTC 2019
New revision: 344123
URL: https://svnweb.freebsd.org/changeset/base/344123

Log:
  [PPC64] Fix mismatch between thread flags and MSR

  When sigreturn() restored a thread's context, SRR1 was being restored
  to its previous value, but pcb_flags was not being touched.

  This could cause a mismatch between the thread's MSR and its pcb_flags.
  For instance, when the thread used the FPU for the first time inside
  the signal handler, sigreturn() would clear SRR1, but not pcb_flags.
  Then, the thread would return with the FPU bit cleared in MSR and,
  the next time it tried to use the FPU, it would fail on a KASSERT
  that checked if the FPU was disabled.

  This change clears the FPU bit in both pcb_flags and frame->srr1,
  as the code that restores the context expects to use the FPU trap
  to re-enable it.

  PR:           234539
  Reported by:  sbruno
  Reviewed by:  jhibbits, sbruno
  Differential Revision:        https://reviews.freebsd.org/D19166

Changes:
  head/sys/powerpc/powerpc/exec_machdep.c

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