[PATCH v3] powerpc: select ARCH_HAS_MEMBARRIER_SYNC_CORE

[PATCH v3] powerpc: select ARCH_HAS_MEMBARRIER_SYNC_CORE

[Nicholas Piggin]

powerpc return from interrupt and return from system call sequences are
context synchronising.

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

v3: more comment fixes

 .../features/sched/membarrier-sync-core/arch-support.txt  | 4 ++--
 arch/powerpc/Kconfig                                      | 1 +
 arch/powerpc/include/asm/exception-64e.h                  | 6 +++++-
 arch/powerpc/include/asm/exception-64s.h                  | 8 ++++++++
 arch/powerpc/kernel/entry_32.S                            | 6 ++++++
 5 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/Documentation/features/sched/membarrier-sync-core/arch-support.txt b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
index 8a521a622966..52ad74a25f54 100644
--- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt
+++ b/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 |
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 9fa23eb320ff..920c4e3ca4ef 100644
--- a/arch/powerpc/Kconfig
+++ b/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
diff --git a/arch/powerpc/include/asm/exception-64e.h b/arch/powerpc/include/asm/exception-64e.h
index 54a98ef7d7fe..72b6657acd2d 100644
--- a/arch/powerpc/include/asm/exception-64e.h
+++ b/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
 
diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
index 47bd4ea0837d..d7a1a427a690 100644
--- a/arch/powerpc/include/asm/exception-64s.h
+++ b/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;					\
diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 217ebdf5b00b..f4d0af8e1136 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/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
-- 
2.23.0

Re: [PATCH v3] powerpc: select ARCH_HAS_MEMBARRIER_SYNC_CORE

[Michael Ellerman]

On Thu, 16 Jul 2020 11:35:22 +1000, Nicholas Piggin wrote:
> powerpc return from interrupt and return from system call sequences are
> context synchronising.

Applied to powerpc/next.

[1/1] powerpc: Select ARCH_HAS_MEMBARRIER_SYNC_CORE
      https://git.kernel.org/powerpc/c/2384b36f9156c3b815a5ce5f694edc5054ab7625

cheers