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) to go back 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@xxxxxxxxxxx>
Signed-off-by: Andrea Parri <parri.andrea@xxxxxxxxx>
---
.../membarrier-sync-core/arch-support.txt | 18 +++++++++++-
MAINTAINERS | 1 +
arch/riscv/Kconfig | 3 ++
arch/riscv/include/asm/membarrier.h | 19 ++++++++++++
arch/riscv/include/asm/sync_core.h | 29 +++++++++++++++++++
kernel/sched/core.c | 4 +++
kernel/sched/membarrier.c | 4 +++
7 files changed, 77 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 d96b778b87ed8..a163170fc0f48 100644
--- a/Documentation/features/sched/membarrier-sync-core/arch-support.txt
+++ b/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) to go back 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 |
diff --git a/MAINTAINERS b/MAINTAINERS
index 6bce0aeecb4f2..e4ca6288ea3d1 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -13817,6 +13817,7 @@ L: linux-kernel@xxxxxxxxxxxxxxx
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
diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 33d9ea5fa392f..2ad63a216d69a 100644
--- a/arch/riscv/Kconfig
+++ b/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
diff --git a/arch/riscv/include/asm/membarrier.h b/arch/riscv/include/asm/membarrier.h
index 6c016ebb5020a..47b240d0d596a 100644
--- a/arch/riscv/include/asm/membarrier.h
+++ b/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.
*/
diff --git a/arch/riscv/include/asm/sync_core.h b/arch/riscv/include/asm/sync_core.h
new file mode 100644
index 0000000000000..9153016da8f14
--- /dev/null
+++ b/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 */
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b51bc86f8340c..82de2b7d253cd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6682,6 +6682,10 @@ 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;
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 6d1f31b3a967b..703e8d80a576d 100644
--- a/kernel/sched/membarrier.c
+++ b/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. */
--
2.34.1
