Revert "powerpc/64s/idle: POWER9 ESL=0 stop avoid save/restore overhead"

As described in that commit:

  When stop is executed with EC=ESL=0, it appears to execute like a
  normal instruction (resuming from NIP when woken by interrupt). So
  all the save/restore handling can be avoided completely.

This is true, except in the case of an NMI interrupt (sreset or
machine check) interrupting the instruction. In that case, the NMI
gets an "interrupt occurred while the processor was in power-saving
mode" indication. The power-save wakeup code uses that bit to decide
whether to restore some registers (e.g., LR). Because these are no
longer saved, this causes random register corruption.

It may be possible to restore this optimisation by detecting the case
of no register loss on the wakeup side, and avoid restoring in that
case, but that's not a minor fix because the wakeup code itself uses
some registers that would be live (e.g., LR).

Fixes: b9ee31e100e7 ("powerpc/64s/idle: POWER9 ESL=0 stop avoid save/restore overhead")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/idle_book3s.S

@@ -324,8 +324,32 @@ enter_winkle:
 /*
  * r3 - PSSCR value corresponding to the requested stop state.
  */
-power_enter_stop_esl:
+power_enter_stop:
+/*
+ * Check if we are executing the lite variant with ESL=EC=0
+ */
+	andis.   r4,r3,PSSCR_EC_ESL_MASK_SHIFTED
 	clrldi   r3,r3,60 /* r3 = Bits[60:63] = Requested Level (RL) */
+	bne	 .Lhandle_esl_ec_set
+	PPC_STOP
+	li	r3,0  /* Since we didn't lose state, return 0 */
+	std	r3, PACA_REQ_PSSCR(r13)
+
+	/*
+	 * pnv_wakeup_noloss() expects r12 to contain the SRR1 value so
+	 * it can determine if the wakeup reason is an HMI in
+	 * CHECK_HMI_INTERRUPT.
+	 *
+	 * However, when we wakeup with ESL=0, SRR1 will not contain the wakeup
+	 * reason, so there is no point setting r12 to SRR1.
+	 *
+	 * Further, we clear r12 here, so that we don't accidentally enter the
+	 * HMI in pnv_wakeup_noloss() if the value of r12[42:45] == WAKE_HMI.
+	 */
+	li	r12, 0
+	b 	pnv_wakeup_noloss
+
+.Lhandle_esl_ec_set:
 BEGIN_FTR_SECTION
 	/*
 	 * POWER9 DD2.0 or earlier can incorrectly set PMAO when waking up after
@@ -417,32 +441,21 @@ _GLOBAL(power9_offline_stop)
 	/* fall through */
 
 _GLOBAL(power9_idle_stop)
-	mtspr 	SPRN_PSSCR,r3
-	/*
-	 * The ESL=EC=0 case does not wake up at 0x100, and it does not
-	 * allow SMT mode switching, so it does not require PSSCR to be
-	 * saved.
-	 */
-	andis.	r4,r3,PSSCR_EC_ESL_MASK_SHIFTED
-	bne	1f
-	PPC_STOP
-	li	r3,0  /* Since we didn't lose state, return 0 */
-	blr
-1:
 	std	r3, PACA_REQ_PSSCR(r13)
 #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
 BEGIN_FTR_SECTION
 	sync
 	lwz	r5, PACA_DONT_STOP(r13)
 	cmpwi	r5, 0
-	bne	2f
+	bne	1f
 END_FTR_SECTION_IFSET(CPU_FTR_P9_TM_XER_SO_BUG)
 #endif
-	LOAD_REG_ADDR(r4,power_enter_stop_esl)
+	mtspr 	SPRN_PSSCR,r3
+	LOAD_REG_ADDR(r4,power_enter_stop)
 	b	pnv_powersave_common
 	/* No return */
 #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
-2:
+1:
 	/*
 	 * We get here when TM / thread reconfiguration bug workaround
 	 * code wants to get the CPU into SMT4 mode, and therefore