powerpc: powernv: Return to cpu offline loop when finished in KVM guest

When a secondary hardware thread has finished running a KVM guest, we currently put that thread into nap mode using a nap instruction in the KVM code. This changes the code so that instead of doing a nap instruction directly, we instead cause the call to power7_nap() that put the thread into nap mode to return. The reason for doing this is to avoid having the KVM code having to know what low-power mode to put the thread into.

In the case of a secondary thread used to run a KVM guest, the thread will be offline from the point of view of the host kernel, and the relevant power7_nap() call is the one in pnv_smp_cpu_disable(). In this case we don't want to clear pending IPIs in the offline loop in that function, since that might cause us to miss the wakeup for the next time the thread needs to run a guest. To tell whether or not to clear the interrupt, we use the SRR1 value returned from power7_nap(), and check if it indicates an external interrupt. We arrange that the return from power7_nap() when we have finished running a guest returns 0, so pending interrupts don't get flushed in that case.

Note that it is important a secondary thread that has finished executing in the guest, or that didn't have a guest to run, should not return to power7_nap's caller while the kvm_hstate.hwthread_req flag in the PACA is non-zero, because the return from power7_nap will reenable the MMU, and the MMU might still be in guest context. In this situation we spin at low priority in real mode waiting for hwthread_req to become zero.

---
I think this would be best going through the powerpc tree. Alex, if you can give me an acked-by for this that would be appreciated.

 arch/powerpc/include/asm/processor.h    |  2 +-
 arch/powerpc/kernel/exceptions-64s.S    |  2 ++
 arch/powerpc/kernel/idle_power7.S       | 12 ++++++--
 arch/powerpc/kvm/book3s_hv_rmhandlers.S | 54 ++++++++++++++++++++++-----------
 arch/powerpc/platforms/powernv/smp.c    | 23 +++++++++++---
 5 files changed, 68 insertions(+), 25 deletions(-)

index dda7ac4..29c3798 100644
@@ -451,7 +451,7 @@ extern unsigned long cpuidle_disable;
 enum idle_boot_override {IDLE_NO_OVERRIDE = 0, IDLE_POWERSAVE_OFF};
 
 extern int powersave_nap;	/* set if nap mode can be used in idle loop */
-extern void power7_nap(int check_irq);
+extern unsigned long power7_nap(int check_irq);
 extern void power7_sleep(void);
 extern void flush_instruction_cache(void);
 extern void hard_reset_now(void);

index a1d45c1..7f29c5f 100644
@@ -131,6 +131,8 @@ BEGIN_FTR_SECTION
 1:
 #endif
 
+	/* Return SRR1 from power7_nap() */
+	mfspr	r3,SPRN_SRR1
 	beq	cr1,2f
 	b	power7_wakeup_noloss
 2:	b	power7_wakeup_loss

index c0754bb..18c0687 100644
@@ -212,6 +212,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_HVMODE)
 	mtspr	SPRN_SRR0,r5
 	rfid
 
+/*
+ * R3 here contains the value that will be returned to the caller
+ * of power7_nap.
+ */
 _GLOBAL(power7_wakeup_loss)
 	ld	r1,PACAR1(r13)
 BEGIN_FTR_SECTION

@@ -219,15 +223,19 @@ BEGIN_FTR_SECTION
 END_FTR_SECTION_IFSET(CPU_FTR_HVMODE)
 	REST_NVGPRS(r1)
 	REST_GPR(2, r1)
-	ld	r3,_CCR(r1)
+	ld	r6,_CCR(r1)
 	ld	r4,_MSR(r1)
 	ld	r5,_NIP(r1)
 	addi	r1,r1,INT_FRAME_SIZE
-	mtcr	r3
+	mtcr	r6
 	mtspr	SPRN_SRR1,r4
 	mtspr	SPRN_SRR0,r5
 	rfid
 
+/*
+ * R3 here contains the value that will be returned to the caller
+ * of power7_nap.
+ */
 _GLOBAL(power7_wakeup_noloss)
 	lbz	r0,PACA_NAPSTATELOST(r13)
 	cmpwi	r0,0

index 4753958..b716f66 100644
@@ -149,6 +149,7 @@ static int pnv_smp_cpu_disable(void)
 static void pnv_smp_cpu_kill_self(void)
 {
 	unsigned int cpu;
+	unsigned long srr1;
 
 	/* Standard hot unplug procedure */
 	local_irq_disable();

@@ -165,13 +166,25 @@ static void pnv_smp_cpu_kill_self(void)
 	mtspr(SPRN_LPCR, mfspr(SPRN_LPCR) & ~(u64)LPCR_PECE1);
 	while (!generic_check_cpu_restart(cpu)) {
 		ppc64_runlatch_off();
-		power7_nap(1);
+		srr1 = power7_nap(1);
 		ppc64_runlatch_on();
 
-		/* Clear the IPI that woke us up */
-		icp_native_flush_interrupt();
-		local_paca->irq_happened &= PACA_IRQ_HARD_DIS;
-		mb();
+		/*
+		 * If the SRR1 value indicates that we woke up due to
+		 * an external interrupt, then clear the interrupt.
+		 * We clear the interrupt before checking for the
+		 * reason, so as to avoid a race where we wake up for
+		 * some other reason, find nothing and clear the interrupt
+		 * just as some other cpu is sending us an interrupt.
+		 * If we returned from power7_nap as a result of
+		 * having finished executing in a KVM guest, then srr1
+		 * contains 0.
+		 */
+		if ((srr1 & SRR1_WAKEMASK) == SRR1_WAKEEE) {
+			icp_native_flush_interrupt();
+			local_paca->irq_happened &= PACA_IRQ_HARD_DIS;
+			smp_mb();
+		}
 
 		if (cpu_core_split_required())
 			continue;

2.1.3

Alex

arch/powerpc/kvm/built-in.o: In function `kvm_no_guest':
arch/powerpc/kvm/book3s_hv_rmhandlers.o:(.text+0x724): undefined reference to `power7_wakeup_loss'

Andreas.

Andreas Schwab, [email protected]
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Ugh. We just removed support for 970 HV mode, but that obviously doesn't mean you can't compile in support for HV mode without enabling p7.

Paul, what would you think of a patch that makes BOOK3S_HV depend on PPC_POWERNV?

Alex

Paul's out until after Christmas I think, maybe later.

That sounds reasonable to me though, it reflects reality, and we can always change it in future.

cheers