powerpc: Select ARCH_HAS_MEMBARRIER_SYNC_CORE

powerpc return from interrupt and return from system call sequences
are context synchronising.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200716013522.338318-1-npiggin@gmail.com

Documentation/features/sched/membarrier-sync-core/arch-support.txt

@@ -5,7 +5,7 @@
 #
 # Architecture requirements
 #
-# * arm/arm64
+# * arm/arm64/powerpc
 #
 # Rely on implicit context synchronization as a result of exception return
 # when returning from IPI handler, and when returning to user-space.
@@ -45,7 +45,7 @@
     |       nios2: | TODO |
     |    openrisc: | TODO |
     |      parisc: | TODO |
-    |     powerpc: | TODO |
+    |     powerpc: |  ok  |
     |       riscv: | TODO |
     |        s390: | TODO |
     |          sh: | TODO |

arch/powerpc/Kconfig

@@ -131,6 +131,7 @@ config PPC
 	select ARCH_HAS_PTE_DEVMAP		if PPC_BOOK3S_64
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_MEMBARRIER_CALLBACKS
+	select ARCH_HAS_MEMBARRIER_SYNC_CORE
 	select ARCH_HAS_SCALED_CPUTIME		if VIRT_CPU_ACCOUNTING_NATIVE && PPC_BOOK3S_64
 	select ARCH_HAS_STRICT_KERNEL_RWX	if (PPC32 && !HIBERNATION)
 	select ARCH_HAS_TICK_BROADCAST		if GENERIC_CLOCKEVENTS_BROADCAST

arch/powerpc/include/asm/exception-64e.h

@@ -204,7 +204,11 @@ exc_##label##_book3e:
 	LOAD_REG_ADDR(r3,interrupt_base_book3e);\
 	ori	r3,r3,vector_offset@l;		\
 	mtspr	SPRN_IVOR##vector_number,r3;
-
+/*
+ * powerpc relies on return from interrupt/syscall being context synchronising
+ * (which rfi is) to support ARCH_HAS_MEMBARRIER_SYNC_CORE without additional
+ * synchronisation instructions.
+ */
 #define RFI_TO_KERNEL							\
 	rfi
 

arch/powerpc/include/asm/exception-64s.h

@@ -68,6 +68,14 @@
  *
  * The nop instructions allow us to insert one or more instructions to flush the
  * L1-D cache when returning to userspace or a guest.
+ *
+ * powerpc relies on return from interrupt/syscall being context synchronising
+ * (which hrfid, rfid, and rfscv are) to support ARCH_HAS_MEMBARRIER_SYNC_CORE
+ * without additional synchronisation instructions.
+ *
+ * soft-masked interrupt replay does not include a context-synchronising rfid,
+ * but those always return to kernel, the sync is only required when returning
+ * to user.
  */
 #define RFI_FLUSH_SLOT							\
 	RFI_FLUSH_FIXUP_SECTION;					\

arch/powerpc/kernel/entry_32.S

@@ -35,6 +35,12 @@
 
 #include "head_32.h"
 
+/*
+ * powerpc relies on return from interrupt/syscall being context synchronising
+ * (which rfi is) to support ARCH_HAS_MEMBARRIER_SYNC_CORE without additional
+ * synchronisation instructions.
+ */
+
 /*
  * Align to 4k in order to ensure that all functions modyfing srr0/srr1
  * fit into one page in order to not encounter a TLB miss between the