[PATCH v3 4/4] powerpc/64s: idle ESL=0 stop can avoid MSR and save/restore overhead

[Nicholas Piggin <npiggin@gmail.com>]

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. In
particular NV GPRs do not have to be saved, and MSR does not have
to be switched back to kernel MSR.

So move the test for "lite" sleep states out to power9_idle_stop.

Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kernel/idle_book3s.S | 35 ++++++++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/kernel/idle_book3s.S b/arch/powerpc/kernel/idle_book3s.S
index 32d65ee323a0..fa56120bd0bc 100644
--- a/arch/powerpc/kernel/idle_book3s.S
+++ b/arch/powerpc/kernel/idle_book3s.S
@@ -315,9 +315,6 @@ enter_winkle:
 
 	ARCH207_IDLE_STATE_ENTER_SEQ_NORET(PPC_WINKLE)
 
-/*
- * r3 - PSSCR value corresponding to the requested stop state.
- */
 #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
 power_enter_stop_kvm_rm:
 	/*
@@ -330,14 +327,11 @@ power_enter_stop_kvm_rm:
 	li	r4,KVM_HWTHREAD_IN_IDLE
 	/* DO THIS IN REAL MODE!  See comment above. */
 	stb	r4,HSTATE_HWTHREAD_STATE(r13)
-#endif
-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
+	andis.	r4,r3,PSSCR_EC_ESL_MASK_SHIFTED
+	bne	power_enter_stop_esl
 	PPC_STOP
 	li	r3,0  /* Since we didn't lose state, return 0 */
 
@@ -354,8 +348,13 @@ power_enter_stop:
 	 */
 	li	r12, 0
 	b 	pnv_wakeup_noloss
+#endif
 
-.Lhandle_esl_ec_set:
+/*
+ * r3 - PSSCR value corresponding to the requested stop state.
+ */
+power_enter_stop_esl:
+	clrldi   r3,r3,60 /* r3 = Bits[60:63] = Requested Level (RL) */
 	/*
 	 * POWER9 DD2 can incorrectly set PMAO when waking up after a
 	 * state-loss idle. Saving and restoring MMCR0 over idle is a
@@ -428,9 +427,23 @@ ALT_FTR_SECTION_END_NESTED_IFSET(CPU_FTR_ARCH_207S, 66);		\
  * r3 contains desired PSSCR register value.
  */
 _GLOBAL(power9_idle_stop)
-	std	r3, PACA_REQ_PSSCR(r13)
 	mtspr 	SPRN_PSSCR,r3
-	LOAD_REG_ADDR(r4,power_enter_stop)
+
+	/*
+	 * Check if we are executing the lite variant with ESL=EC=0
+	 * This case resumes execution after the stop instruction without
+	 * losing any state, so nothing has to be saved. The following
+	 * instructions up to the blr must be skipped if we want to
+	 * use power_enter_stop_kvm_rm.
+	 */
+	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:	/* state-loss idle */
+	std	r3, PACA_REQ_PSSCR(r13)
+	LOAD_REG_ADDR(r4,power_enter_stop_esl)
 	b	pnv_powersave_common
 	/* No return */
 
-- 
2.13.3