powerpc fixes for 5.15 #5

Fix a bug exposed by a previous fix, where running guests with certain SMT topologies
 could crash the host on Power8.
 
 Fix atomic sleep warnings when re-onlining CPUs, when PREEMPT is enabled.
 
 Thanks to: Nathan Lynch, Srikar Dronamraju, Valentin Schneider.
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Merge tag 'powerpc-5.15-5' of git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux

Pull powerpc fixes from Michael Ellerman:

 - Fix a bug exposed by a previous fix, where running guests with
   certain SMT topologies could crash the host on Power8.

 - Fix atomic sleep warnings when re-onlining CPUs, when PREEMPT is
   enabled.

Thanks to Nathan Lynch, Srikar Dronamraju, and Valentin Schneider.

* tag 'powerpc-5.15-5' of git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux:
  powerpc/smp: do not decrement idle task preempt count in CPU offline
  powerpc/idle: Don't corrupt back chain when going idle

arch/powerpc/kernel/idle_book3s.S

@@ -126,14 +126,16 @@ _GLOBAL(idle_return_gpr_loss)
 /*
  * This is the sequence required to execute idle instructions, as
  * specified in ISA v2.07 (and earlier). MSR[IR] and MSR[DR] must be 0.
- *
- * The 0(r1) slot is used to save r2 in isa206, so use that here.
+ * We have to store a GPR somewhere, ptesync, then reload it, and create
+ * a false dependency on the result of the load. It doesn't matter which
+ * GPR we store, or where we store it. We have already stored r2 to the
+ * stack at -8(r1) in isa206_idle_insn_mayloss, so use that.
  */
 #define IDLE_STATE_ENTER_SEQ_NORET(IDLE_INST)			\
 	/* Magic NAP/SLEEP/WINKLE mode enter sequence */	\
-	std	r2,0(r1);					\
+	std	r2,-8(r1);					\
 	ptesync;						\
-	ld	r2,0(r1);					\
+	ld	r2,-8(r1);					\
 236:	cmpd	cr0,r2,r2;					\
 	bne	236b;						\
 	IDLE_INST;						\

arch/powerpc/kernel/smp.c

@@ -1730,8 +1730,6 @@ void __cpu_die(unsigned int cpu)
 
 void arch_cpu_idle_dead(void)
 {
-	sched_preempt_enable_no_resched();
-
 	/*
 	 * Disable on the down path. This will be re-enabled by
 	 * start_secondary() via start_secondary_resume() below