powerpc/64s/interrupt: avoid saving CFAR in some asynchronous interrupts

Reading the CFAR register is quite costly (~20 cycles on POWER9). It is
a good idea to have for most synchronous interrupts, but for async ones
it is much less important.

Doorbell, external, and decrementer interrupts are the important
asynchronous ones. HV interrupts can't skip CFAR if KVM HV is possible,
because it might be a guest exit that requires CFAR preserved. But the
important pseries interrupts can avoid loading CFAR.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210922145452.352571-7-npiggin@gmail.com

arch/powerpc/kernel/exceptions-64s.S

@@ -111,6 +111,8 @@ name:
 #define IAREA		.L_IAREA_\name\()	/* PACA save area */
 #define IVIRT		.L_IVIRT_\name\()	/* Has virt mode entry point */
 #define IISIDE		.L_IISIDE_\name\()	/* Uses SRR0/1 not DAR/DSISR */
+#define ICFAR		.L_ICFAR_\name\()	/* Uses CFAR */
+#define ICFAR_IF_HVMODE	.L_ICFAR_IF_HVMODE_\name\() /* Uses CFAR if HV */
 #define IDAR		.L_IDAR_\name\()	/* Uses DAR (or SRR0) */
 #define IDSISR		.L_IDSISR_\name\()	/* Uses DSISR (or SRR1) */
 #define IBRANCH_TO_COMMON	.L_IBRANCH_TO_COMMON_\name\() /* ENTRY branch to common */
@@ -150,6 +152,12 @@ do_define_int n
 	.ifndef IISIDE
 		IISIDE=0
 	.endif
+	.ifndef ICFAR
+		ICFAR=1
+	.endif
+	.ifndef ICFAR_IF_HVMODE
+		ICFAR_IF_HVMODE=0
+	.endif
 	.ifndef IDAR
 		IDAR=0
 	.endif
@@ -287,9 +295,21 @@ BEGIN_FTR_SECTION
 END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 	HMT_MEDIUM
 	std	r10,IAREA+EX_R10(r13)		/* save r10 - r12 */
+	.if ICFAR
 BEGIN_FTR_SECTION
 	mfspr	r10,SPRN_CFAR
 END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
+	.elseif ICFAR_IF_HVMODE
+BEGIN_FTR_SECTION
+  BEGIN_FTR_SECTION_NESTED(69)
+	mfspr	r10,SPRN_CFAR
+  END_FTR_SECTION_NESTED(CPU_FTR_CFAR, CPU_FTR_CFAR, 69)
+FTR_SECTION_ELSE
+  BEGIN_FTR_SECTION_NESTED(69)
+	li	r10,0
+  END_FTR_SECTION_NESTED(CPU_FTR_CFAR, CPU_FTR_CFAR, 69)
+ALT_FTR_SECTION_END_IFSET(CPU_FTR_HVMODE | CPU_FTR_ARCH_206)
+	.endif
 	.if \ool
 	.if !\virt
 	b	tramp_real_\name
@@ -305,9 +325,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
 BEGIN_FTR_SECTION
 	std	r9,IAREA+EX_PPR(r13)
 END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
+	.if ICFAR || ICFAR_IF_HVMODE
 BEGIN_FTR_SECTION
 	std	r10,IAREA+EX_CFAR(r13)
 END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
+	.endif
 	INTERRUPT_TO_KERNEL
 	mfctr	r10
 	std	r10,IAREA+EX_CTR(r13)
@@ -559,7 +581,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
 	.endif
 
 BEGIN_FTR_SECTION
+	.if ICFAR || ICFAR_IF_HVMODE
 	ld	r10,IAREA+EX_CFAR(r13)
+	.else
+	li	r10,0
+	.endif
 	std	r10,ORIG_GPR3(r1)
 END_FTR_SECTION_IFSET(CPU_FTR_CFAR)
 	ld	r10,IAREA+EX_CTR(r13)
@@ -1520,6 +1546,12 @@ ALT_MMU_FTR_SECTION_END_IFCLR(MMU_FTR_TYPE_RADIX)
  *
  * If soft masked, the masked handler will note the pending interrupt for
  * replay, and clear MSR[EE] in the interrupted context.
+ *
+ * CFAR is not required because this is an asynchronous interrupt that in
+ * general won't have much bearing on the state of the CPU, with the possible
+ * exception of crash/debug IPIs, but those are generally moving to use SRESET
+ * IPIs. Unless this is an HV interrupt and KVM HV is possible, in which case
+ * it may be exiting the guest and need CFAR to be saved.
  */
 INT_DEFINE_BEGIN(hardware_interrupt)
 	IVEC=0x500
@@ -1527,6 +1559,10 @@ INT_DEFINE_BEGIN(hardware_interrupt)
 	IMASK=IRQS_DISABLED
 	IKVM_REAL=1
 	IKVM_VIRT=1
+	ICFAR=0
+#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
+	ICFAR_IF_HVMODE=1
+#endif
 INT_DEFINE_END(hardware_interrupt)
 
 EXC_REAL_BEGIN(hardware_interrupt, 0x500, 0x100)
@@ -1748,6 +1784,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_TM)
  * If PPC_WATCHDOG is configured, the soft masked handler will actually set
  * things back up to run soft_nmi_interrupt as a regular interrupt handler
  * on the emergency stack.
+ *
+ * CFAR is not required because this is asynchronous (see hardware_interrupt).
+ * A watchdog interrupt may like to have CFAR, but usually the interesting
+ * branch is long gone by that point (e.g., infinite loop).
  */
 INT_DEFINE_BEGIN(decrementer)
 	IVEC=0x900
@@ -1755,6 +1795,7 @@ INT_DEFINE_BEGIN(decrementer)
 #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
 	IKVM_REAL=1
 #endif
+	ICFAR=0
 INT_DEFINE_END(decrementer)
 
 EXC_REAL_BEGIN(decrementer, 0x900, 0x80)
@@ -1830,6 +1871,8 @@ EXC_COMMON_BEGIN(hdecrementer_common)
  * If soft masked, the masked handler will note the pending interrupt for
  * replay, leaving MSR[EE] enabled in the interrupted context because the
  * doorbells are edge triggered.
+ *
+ * CFAR is not required, similarly to hardware_interrupt.
  */
 INT_DEFINE_BEGIN(doorbell_super)
 	IVEC=0xa00
@@ -1837,6 +1880,7 @@ INT_DEFINE_BEGIN(doorbell_super)
 #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
 	IKVM_REAL=1
 #endif
+	ICFAR=0
 INT_DEFINE_END(doorbell_super)
 
 EXC_REAL_BEGIN(doorbell_super, 0xa00, 0x100)
@@ -1888,6 +1932,7 @@ INT_DEFINE_BEGIN(system_call)
 	IVEC=0xc00
 	IKVM_REAL=1
 	IKVM_VIRT=1
+	ICFAR=0
 INT_DEFINE_END(system_call)
 
 .macro SYSTEM_CALL virt
@@ -2186,6 +2231,11 @@ EXC_COMMON_BEGIN(hmi_exception_common)
  * Interrupt 0xe80 - Directed Hypervisor Doorbell Interrupt.
  * This is an asynchronous interrupt in response to a msgsnd doorbell.
  * Similar to the 0xa00 doorbell but for host rather than guest.
+ *
+ * CFAR is not required (similar to doorbell_interrupt), unless KVM HV
+ * is enabled, in which case it may be a guest exit. Most PowerNV kernels
+ * include KVM support so it would be nice if this could be dynamically
+ * patched out if KVM was not currently running any guests.
  */
 INT_DEFINE_BEGIN(h_doorbell)
 	IVEC=0xe80
@@ -2193,6 +2243,9 @@ INT_DEFINE_BEGIN(h_doorbell)
 	IMASK=IRQS_DISABLED
 	IKVM_REAL=1
 	IKVM_VIRT=1
+#ifndef CONFIG_KVM_BOOK3S_HV_POSSIBLE
+	ICFAR=0
+#endif
 INT_DEFINE_END(h_doorbell)
 
 EXC_REAL_BEGIN(h_doorbell, 0xe80, 0x20)
@@ -2216,6 +2269,9 @@ EXC_COMMON_BEGIN(h_doorbell_common)
  * Interrupt 0xea0 - Hypervisor Virtualization Interrupt.
  * This is an asynchronous interrupt in response to an "external exception".
  * Similar to 0x500 but for host only.
+ *
+ * Like h_doorbell, CFAR is only required for KVM HV because this can be
+ * a guest exit.
  */
 INT_DEFINE_BEGIN(h_virt_irq)
 	IVEC=0xea0
@@ -2223,6 +2279,9 @@ INT_DEFINE_BEGIN(h_virt_irq)
 	IMASK=IRQS_DISABLED
 	IKVM_REAL=1
 	IKVM_VIRT=1
+#ifndef CONFIG_KVM_BOOK3S_HV_POSSIBLE
+	ICFAR=0
+#endif
 INT_DEFINE_END(h_virt_irq)
 
 EXC_REAL_BEGIN(h_virt_irq, 0xea0, 0x20)
@@ -2259,6 +2318,8 @@ EXC_VIRT_NONE(0x4ee0, 0x20)
  *
  * If soft masked, the masked handler will note the pending interrupt for
  * replay, and clear MSR[EE] in the interrupted context.
+ *
+ * CFAR is not used by perf interrupts so not required.
  */
 INT_DEFINE_BEGIN(performance_monitor)
 	IVEC=0xf00
@@ -2266,6 +2327,7 @@ INT_DEFINE_BEGIN(performance_monitor)
 #ifdef CONFIG_KVM_BOOK3S_PR_POSSIBLE
 	IKVM_REAL=1
 #endif
+	ICFAR=0
 INT_DEFINE_END(performance_monitor)
 
 EXC_REAL_BEGIN(performance_monitor, 0xf00, 0x20)
@@ -2690,6 +2752,7 @@ EXC_VIRT_NONE(0x5800, 0x100)
 INT_DEFINE_BEGIN(soft_nmi)
 	IVEC=0x900
 	ISTACK=0
+	ICFAR=0
 INT_DEFINE_END(soft_nmi)
 
 /*