[PATCH v3 1/4] powerpc: powernv: Switch off MMU before entering nap/sleep/rvwinkle mode

From: Shreyas B. Prabhu
Date: Thu Dec 04 2014 - 02:29:08 EST

From: Paul Mackerras <paulus@xxxxxxxxx>

Currently, when going idle, we set the flag indicating that we are in
nap mode (paca->kvm_hstate.hwthread_state) and then execute the nap
(or sleep or rvwinkle) instruction, all with the MMU on. This is bad
for two reasons: (a) the architecture specifies that those instructions
must be executed with the MMU off, and in fact with only the SF, HV, ME
and possibly RI bits set, and (b) this introduces a race, because as
soon as we set the flag, another thread can switch the MMU to a guest
context. If the race is lost, this thread will typically start looping
on relocation-on ISIs at 0xc...4400.

This fixes it by setting the MSR as required by the architecture before
setting the flag or executing the nap/sleep/rvwinkle instruction.

[ shreyas@xxxxxxxxxxxxxxxxxx: Edited to handle LE ]
Signed-off-by: Paul Mackerras <paulus@xxxxxxxxx>
Signed-off-by: Shreyas B. Prabhu <shreyas@xxxxxxxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
arch/powerpc/include/asm/reg.h | 2 ++
arch/powerpc/kernel/idle_power7.S | 18 +++++++++++++++++-
2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
index c998279..a68ee15 100644
--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -118,8 +118,10 @@
#ifdef __BIG_ENDIAN__
#define MSR_ __MSR
+#define MSR_IDLE (MSR_ME | MSR_SF | MSR_HV)
#define MSR_ (__MSR | MSR_LE)
#define MSR_KERNEL (MSR_ | MSR_64BIT)
#define MSR_USER32 (MSR_ | MSR_PR | MSR_EE)
diff --git a/arch/powerpc/kernel/idle_power7.S b/arch/powerpc/kernel/idle_power7.S
index c0754bb..283c603 100644
--- a/arch/powerpc/kernel/idle_power7.S
+++ b/arch/powerpc/kernel/idle_power7.S
@@ -101,7 +101,23 @@ _GLOBAL(power7_powersave_common)
std r9,_MSR(r1)
std r1,PACAR1(r13)

+ /*
+ * Go to real mode to do the nap, as required by the architecture.
+ * Also, we need to be in real mode before setting hwthread_state,
+ * because as soon as we do that, another thread can switch
+ * the MMU context to the guest.
+ */
+ li r6, MSR_RI
+ andc r6, r9, r6
+ LOAD_REG_ADDR(r7, power7_enter_nap_mode)
+ mtmsrd r6, 1 /* clear RI before setting SRR0/1 */
+ mtspr SPRN_SRR0, r7
+ mtspr SPRN_SRR1, r5
+ rfid
+ .globl power7_enter_nap_mode
/* Tell KVM we're napping */

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