membarrier: riscv: Provide core serializing command

RISC-V uses xRET instructions on return from interrupt and to go back
to user-space; the xRET instruction is not core serializing.

Use FENCE.I for providing core serialization as follows:

 - by calling sync_core_before_usermode() on return from interrupt (cf.
   ipi_sync_core()),

 - via switch_mm() and sync_core_before_usermode() (respectively, for
   uthread->uthread and kthread->uthread transitions) before returning
   to user-space.

On RISC-V, the serialization in switch_mm() is activated by resetting
the icache_stale_mask of the mm at prepare_sync_core_cmd().

Suggested-by: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/r/20240131144936.29190-5-parri.andrea@gmail.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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

@@ -10,6 +10,22 @@
 # Rely on implicit context synchronization as a result of exception return
 # when returning from IPI handler, and when returning to user-space.
 #
+# * riscv
+#
+# riscv uses xRET as return from interrupt and to return to user-space.
+#
+# Given that xRET is not core serializing, we rely on FENCE.I for providing
+# core serialization:
+#
+#  - by calling sync_core_before_usermode() on return from interrupt (cf.
+#    ipi_sync_core()),
+#
+#  - via switch_mm() and sync_core_before_usermode() (respectively, for
+#    uthread->uthread and kthread->uthread transitions) before returning
+#    to user-space.
+#
+#  The serialization in switch_mm() is activated by prepare_sync_core_cmd().
+#
 # * x86
 #
 # x86-32 uses IRET as return from interrupt, which takes care of the IPI.
@@ -43,7 +59,7 @@
     |    openrisc: | TODO |
     |      parisc: | TODO |
     |     powerpc: |  ok  |
-    |       riscv: | TODO |
+    |       riscv: |  ok  |
     |        s390: |  ok  |
     |          sh: | TODO |
     |       sparc: | TODO |

MAINTAINERS

@@ -14041,6 +14041,7 @@ L:	linux-kernel@vger.kernel.org
 S:	Supported
 F:	Documentation/scheduler/membarrier.rst
 F:	arch/*/include/asm/membarrier.h
+F:	arch/*/include/asm/sync_core.h
 F:	include/uapi/linux/membarrier.h
 F:	kernel/sched/membarrier.c
 

arch/riscv/Kconfig

@@ -28,14 +28,17 @@ config RISCV
 	select ARCH_HAS_GIGANTIC_PAGE
 	select ARCH_HAS_KCOV
 	select ARCH_HAS_MEMBARRIER_CALLBACKS
+	select ARCH_HAS_MEMBARRIER_SYNC_CORE
 	select ARCH_HAS_MMIOWB
 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
 	select ARCH_HAS_PMEM_API
+	select ARCH_HAS_PREPARE_SYNC_CORE_CMD
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_SET_DIRECT_MAP if MMU
 	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_SYSCALL_WRAPPER
 	select ARCH_HAS_TICK_BROADCAST if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UBSAN_SANITIZE_ALL

arch/riscv/include/asm/membarrier.h

@@ -22,6 +22,25 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 	/*
 	 * The membarrier system call requires a full memory barrier
 	 * after storing to rq->curr, before going back to user-space.
+	 *
+	 * This barrier is also needed for the SYNC_CORE command when
+	 * switching between processes; in particular, on a transition
+	 * from a thread belonging to another mm to a thread belonging
+	 * to the mm for which a membarrier SYNC_CORE is done on CPU0:
+	 *
+	 *   - [CPU0] sets all bits in the mm icache_stale_mask (in
+	 *     prepare_sync_core_cmd());
+	 *
+	 *   - [CPU1] stores to rq->curr (by the scheduler);
+	 *
+	 *   - [CPU0] loads rq->curr within membarrier and observes
+	 *     cpu_rq(1)->curr->mm != mm, so the IPI is skipped on
+	 *     CPU1; this means membarrier relies on switch_mm() to
+	 *     issue the sync-core;
+	 *
+	 *   - [CPU1] switch_mm() loads icache_stale_mask; if the bit
+	 *     is zero, switch_mm() may incorrectly skip the sync-core.
+	 *
 	 * Matches a full barrier in the proximity of the membarrier
 	 * system call entry.
 	 */

arch/riscv/include/asm/sync_core.h

@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_RISCV_SYNC_CORE_H
+#define _ASM_RISCV_SYNC_CORE_H
+
+/*
+ * 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");
+}
+
+#ifdef CONFIG_SMP
+/*
+ * Ensure the next switch_mm() on every CPU issues a core serializing
+ * instruction for the given @mm.
+ */
+static inline void prepare_sync_core_cmd(struct mm_struct *mm)
+{
+	cpumask_setall(&mm->context.icache_stale_mask);
+}
+#else
+static inline void prepare_sync_core_cmd(struct mm_struct *mm)
+{
+}
+#endif /* CONFIG_SMP */
+
+#endif /* _ASM_RISCV_SYNC_CORE_H */

kernel/sched/core.c

@@ -6721,6 +6721,10 @@ static void __sched notrace __schedule(unsigned int sched_mode)
 		 *
 		 * The barrier matches a full barrier in the proximity of
 		 * the membarrier system call entry.
+		 *
+		 * On RISC-V, this barrier pairing is also needed for the
+		 * SYNC_CORE command when switching between processes, cf.
+		 * the inline comments in membarrier_arch_switch_mm().
 		 */
 		++*switch_count;
 

kernel/sched/membarrier.c

@@ -342,6 +342,10 @@ static int membarrier_private_expedited(int flags, int cpu_id)
 	/*
 	 * Matches memory barriers after rq->curr modification in
 	 * scheduler.
+	 *
+	 * On RISC-V, this barrier pairing is also needed for the
+	 * SYNC_CORE command when switching between processes, cf.
+	 * the inline comments in membarrier_arch_switch_mm().
 	 */
 	smp_mb();	/* system call entry is not a mb. */