Re: [PATCH] powerpc/powernv: Clear LPCR[PECE1] via stop-api only for deep state offline

On 08/31/2017 05:37 PM, Nicholas Piggin wrote:

On Thu, 31 Aug 2017 17:17:41 +0530
"Gautham R. Shenoy" [off-list ref] wrote:

quoted

From: "Gautham R. Shenoy" <redacted>

commit 24be85a23d1f ("powerpc/powernv: Clear PECE1 in LPCR via
stop-api only on Hotplug") clears the PECE1 bit of the LPCR via
stop-api during CPU-Hotplug to prevent wakeup due to a decrementer on
an offlined CPU which is in a deep stop state.

In the case where the stop-api support is found to be lacking, the
commit 785a12afdb4a ("powerpc/powernv/idle: Disable LOSE_FULL_CONTEXT
states when stop-api fails") disables deep states that lose hypervisor
context. Thus in this case, the offlined CPU will be put to some
shallow idle state.

However, we currently unconditionally clear the PECE1 in LPCR via
stop-api during CPU-Hotplug even when deep states are disabled due to
stop-api failure.

Fix this by clearing PECE1 of LPCR via stop-api during CPU-Hotplug
*only* when the offlined CPU will be put to a deep state that loses
hypervisor context.

This looks okay to me. The bug is due to calling opal_slw_set_reg when
firmware has not enabled that feature, right?

Yes,

In the case where the stop-api support is found to be lacking, the 
commit 785a12afdb4a ("powerpc/powernv/idle: Disable LOSE_FULL_CONTEXT 
states when stop-api fails") disables deep states that lose hypervisor 
context. Thus in this case, the offlined CPU will be put to some shallow 
idle state.

If a shallow state ( < stop4 ) is being chosen for cpu hotplug, then :
1) this opal call is not required.
2) may not be supported.

Hence  should call opal_slw_set_reg() only if a deep state chosen for 
cpu hotplug.

quoted

Fixes: commit 24be85a23d1f ("powerpc/powernv: Clear PECE1 in LPCR via
stop-api only on Hotplug")

Reported-by: Pavithra Prakash <redacted>
Signed-off-by: Gautham R. Shenoy <redacted>
---
 arch/powerpc/platforms/powernv/idle.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/powernv/idle.c

b/arch/powerpc/platforms/powernv/idle.c

quoted

index 9f59041..23f8fba 100644

--- a/arch/powerpc/platforms/powernv/idle.c
+++ b/arch/powerpc/platforms/powernv/idle.c

@@ -393,7 +393,13 @@ static void

pnv_program_cpu_hotplug_lpcr(unsigned int cpu, u64 lpcr_val)

quoted

     u64 pir = get_hard_smp_processor_id(cpu);

     mtspr(SPRN_LPCR, lpcr_val);
-    opal_slw_set_reg(pir, SPRN_LPCR, lpcr_val);
+
+    /*
+     * Program the LPCR via stop-api only for deepest stop state
+     * can lose hypervisor context.
+     */
+    if (supported_cpuidle_states & OPAL_PM_LOSE_FULL_CONTEXT)
+        opal_slw_set_reg(pir, SPRN_LPCR, lpcr_val);
 }

 /*

On Fri, 1 Sep 2017 15:38:59 +0530
Akshay Adiga [off-list ref] wrote:

On 08/31/2017 05:37 PM, Nicholas Piggin wrote:

quoted

On Thu, 31 Aug 2017 17:17:41 +0530
"Gautham R. Shenoy" [off-list ref] wrote:
 

quoted

From: "Gautham R. Shenoy" <redacted>

commit 24be85a23d1f ("powerpc/powernv: Clear PECE1 in LPCR via
stop-api only on Hotplug") clears the PECE1 bit of the LPCR via
stop-api during CPU-Hotplug to prevent wakeup due to a decrementer on
an offlined CPU which is in a deep stop state.

In the case where the stop-api support is found to be lacking, the
commit 785a12afdb4a ("powerpc/powernv/idle: Disable LOSE_FULL_CONTEXT
states when stop-api fails") disables deep states that lose hypervisor
context. Thus in this case, the offlined CPU will be put to some
shallow idle state.

However, we currently unconditionally clear the PECE1 in LPCR via
stop-api during CPU-Hotplug even when deep states are disabled due to
stop-api failure.

Fix this by clearing PECE1 of LPCR via stop-api during CPU-Hotplug
*only* when the offlined CPU will be put to a deep state that loses
hypervisor context.  

This looks okay to me. The bug is due to calling opal_slw_set_reg when
firmware has not enabled that feature, right?  

Yes,

In the case where the stop-api support is found to be lacking, the 
commit 785a12afdb4a ("powerpc/powernv/idle: Disable LOSE_FULL_CONTEXT 
states when stop-api fails") disables deep states that lose hypervisor 
context. Thus in this case, the offlined CPU will be put to some shallow 
idle state.

If a shallow state ( < stop4 ) is being chosen for cpu hotplug, then :
1) this opal call is not required.
2) may not be supported.

Hence  should call opal_slw_set_reg() only if a deep state chosen for 
cpu hotplug.

That makes sense, thanks for confirming.

quoted

quoted

Fixes: commit 24be85a23d1f ("powerpc/powernv: Clear PECE1 in LPCR via
stop-api only on Hotplug")

Reported-by: Pavithra Prakash <redacted>
Signed-off-by: Gautham R. Shenoy <redacted>

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

quoted

quoted

---
 arch/powerpc/platforms/powernv/idle.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/powernv/idle.c  

b/arch/powerpc/platforms/powernv/idle.c  

quoted

index 9f59041..23f8fba 100644

--- a/arch/powerpc/platforms/powernv/idle.c
+++ b/arch/powerpc/platforms/powernv/idle.c

@@ -393,7 +393,13 @@ static void  

pnv_program_cpu_hotplug_lpcr(unsigned int cpu, u64 lpcr_val)  

quoted

     u64 pir = get_hard_smp_processor_id(cpu);

     mtspr(SPRN_LPCR, lpcr_val);
-    opal_slw_set_reg(pir, SPRN_LPCR, lpcr_val);
+
+    /*
+     * Program the LPCR via stop-api only for deepest stop state
+     * can lose hypervisor context.
+     */
+    if (supported_cpuidle_states & OPAL_PM_LOSE_FULL_CONTEXT)
+        opal_slw_set_reg(pir, SPRN_LPCR, lpcr_val);
 }

 /*