[RFC PATCH] membarrier: riscv: Provide core serializing command

[Andrea Parri <parri.andrea@gmail.com>]

Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Suggested-by: Palmer Dabbelt <palmer@dabbelt.com>
---
For the MEMBARRIER maintainers:  RISC-V does not have "core serializing
instructions", meaning that there is no occurence of such a term in the
RISC-V ISA.  The discussion and git history about the SYNC_CORE command
suggested the implementation below: a FENCE.I instruction "synchronizes
the instruction and data streams" [1] on a CPU; in litmus parlance,

  (single-hart test)

  CPU0

  UPDATE text   ;
  FENCE.I       ;
  EXECUTE text  ;  /* <-- will execute the updated/new text */


  (message-passing test)

  CPU0             CPU1

  UPDATE text   |  IF (flag) {     ;
  WMB           |    FENCE.I       ;
  SET flag      |    EXECUTE text  ;  /* execute the new text */
                |  }               ;


  (and many others, including "maybe"s!  ;-) )

How do these remarks resonate with the semantics of "a core serializing
instruction" (to be issued before returning to user-space)?

RISCV maintainers, I'm missing some paths to user-space? (besides xRET)

  Andrea

[1] https://github.com/riscv/riscv-isa-manual/blob/main/src/zifencei.adoc


 .../sched/membarrier-sync-core/arch-support.txt   |  2 +-
 arch/riscv/Kconfig                                |  2 ++
 arch/riscv/include/asm/sync_core.h                | 15 +++++++++++++++
 3 files changed, 18 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/include/asm/sync_core.h

diff --git a/Documentation/features/sched/membarrier-sync-core/arch-support.txt b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
index 23260ca449468..a17117d76e6d8 100644
--- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt
+++ b/Documentation/features/sched/membarrier-sync-core/arch-support.txt
@@ -44,7 +44,7 @@
     |    openrisc: | TODO |
     |      parisc: | TODO |
     |     powerpc: |  ok  |
-    |       riscv: | TODO |
+    |       riscv: |  ok  |
     |        s390: |  ok  |
     |          sh: | TODO |
     |       sparc: | TODO |
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 4c07b9189c867..ed7ddaedc692e 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -27,6 +27,7 @@ config RISCV
 	select ARCH_HAS_GCOV_PROFILE_ALL
 	select ARCH_HAS_GIGANTIC_PAGE
 	select ARCH_HAS_KCOV
+	select ARCH_HAS_MEMBARRIER_SYNC_CORE
 	select ARCH_HAS_MMIOWB
 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
 	select ARCH_HAS_PMEM_API
@@ -35,6 +36,7 @@ config RISCV
 	select ARCH_HAS_SET_MEMORY if MMU
 	select ARCH_HAS_STRICT_KERNEL_RWX if MMU && !XIP_KERNEL
 	select ARCH_HAS_STRICT_MODULE_RWX if MMU && !XIP_KERNEL
+	select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
 	select ARCH_HAS_VDSO_DATA
diff --git a/arch/riscv/include/asm/sync_core.h b/arch/riscv/include/asm/sync_core.h
new file mode 100644
index 0000000000000..d3ec6ac47ac9b
--- /dev/null
+++ b/arch/riscv/include/asm/sync_core.h
@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_RISCV_SYNC_CORE_H
+#define _ASM_RISCV_SYNC_CORE_H
+
+/*
+ * Ensure that a core serializing instruction is issued before returning
+ * to user-mode.  RISC-V implements return to user-space through an xRET
+ * instruction, which is not core serializing.
+ */
+static inline void sync_core_before_usermode(void)
+{
+	asm volatile ("fence.i" ::: "memory");
+}
+
+#endif /* _ASM_RISCV_SYNC_CORE_H */
-- 
2.34.1