[RFC PATCH v2 4/6] RISC-V: Implement prctl call to set/get the memory consistency model

[Christoph Müllner]

We can use the PR_{S,G}ET_MEMORY_CONSISTENCY_MODEL prctl calls to change
the memory consistency model at run-time if we have Ssdtso.
This patch registers RISCV_WMO and RISCV_TSO as valid arguments
for these prctl calls and implements the glue code to switch
between these.

Signed-off-by: Christoph Müllner <christoph.muellner at vrull.eu>
---
 .../mm/dynamic-memory-consistency-model.rst   | 12 +++-
 arch/riscv/include/asm/processor.h            |  7 ++
 arch/riscv/kernel/Makefile                    |  1 +
 arch/riscv/kernel/dtso.c                      | 67 +++++++++++++++++++
 include/uapi/linux/prctl.h                    |  2 +
 5 files changed, 88 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/kernel/dtso.c

diff --git a/Documentation/mm/dynamic-memory-consistency-model.rst b/Documentation/mm/dynamic-memory-consistency-model.rst
index 1fce855a1fad..c8188c174e27 100644
--- a/Documentation/mm/dynamic-memory-consistency-model.rst
+++ b/Documentation/mm/dynamic-memory-consistency-model.rst
@@ -73,4 +73,14 @@ Supported memory consistency models
 This section defines the memory consistency models which are supported
 by the prctl interface.
 
-<none>
+RISC-V
+------
+
+RISC-V uses RVWMO (RISC-V weak memory ordering) as default memory consistency
+model. TSO (total store ordering) is another specified model and provides
+additional ordering guarantees. Switching user-mode processes from RVWMO to TSO
+is possible when the Ssdtso extension is available.
+
+* :c:macro:`PR_MEMORY_CONSISTENCY_MODEL_RISCV_WMO`: RISC-V weak memory ordering (default).
+
+* :c:macro:`PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO`: RISC-V total store ordering.
diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index a8509cc31ab2..05e05fddc94d 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -184,6 +184,13 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val);
 #define GET_UNALIGN_CTL(tsk, addr)	get_unalign_ctl((tsk), (addr))
 #define SET_UNALIGN_CTL(tsk, val)	set_unalign_ctl((tsk), (val))
 
+#ifdef CONFIG_RISCV_ISA_SSDTSO
+extern int dtso_set_memory_consistency_model(unsigned long arg);
+extern int dtso_get_memory_consistency_model(void);
+#define SET_MEMORY_CONSISTENCY_MODEL(arg)	dtso_set_memory_consistency_model(arg)
+#define GET_MEMORY_CONSISTENCY_MODEL()		dtso_get_memory_consistency_model()
+#endif /* CONIG_RISCV_ISA_SSDTSO */
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_RISCV_PROCESSOR_H */
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index f71910718053..85f7291da498 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -65,6 +65,7 @@ obj-$(CONFIG_RISCV_MISALIGNED)	+= traps_misaligned.o
 obj-$(CONFIG_FPU)		+= fpu.o
 obj-$(CONFIG_RISCV_ISA_V)	+= vector.o
 obj-$(CONFIG_RISCV_ISA_V)	+= kernel_mode_vector.o
+obj-$(CONFIG_RISCV_ISA_SSDTSO)	+= dtso.o
 obj-$(CONFIG_SMP)		+= smpboot.o
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_SMP)		+= cpu_ops.o
diff --git a/arch/riscv/kernel/dtso.c b/arch/riscv/kernel/dtso.c
new file mode 100644
index 000000000000..591d5f9de0f5
--- /dev/null
+++ b/arch/riscv/kernel/dtso.c
@@ -0,0 +1,67 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (c) 2024 Christoph Muellner <christoph.muellner at vrull.eu>
+ */
+
+#include <linux/cpu.h>
+#include <linux/smp.h>
+#include <linux/prctl.h>
+
+#include <asm/cpu.h>
+#include <asm/dtso.h>
+
+#include <trace/events/ipi.h>
+
+int dtso_set_memory_consistency_model(unsigned long arg)
+{
+	int cpu;
+	unsigned long cur_model = get_memory_consistency_model(current);
+	unsigned long new_model;
+
+	switch (arg) {
+	case PR_MEMORY_CONSISTENCY_MODEL_RISCV_WMO:
+		new_model = RISCV_MEMORY_CONSISTENCY_MODEL_WMO;
+		break;
+	case PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO:
+		new_model = RISCV_MEMORY_CONSISTENCY_MODEL_TSO;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	/* No change requested. */
+	if (cur_model == new_model)
+		return 0;
+
+	/* Enabling TSO only works if DTSO is available. */
+	if (new_model == PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO && !has_dtso())
+		return -EINVAL;
+
+	/* Switching TSO->WMO is not allowed. */
+	if (new_model == RISCV_MEMORY_CONSISTENCY_MODEL_WMO)
+		return -EINVAL;
+
+	/* Set the new model in the task struct. */
+	set_memory_consitency_model(current, new_model);
+
+	/*
+	 * We need to reschedule all threads of the current process.
+	 * Let's do this by rescheduling all CPUs.
+	 * This is stricter than necessary, but since this call is
+	 * not expected to happen frequently the impact is low.
+	 */
+	for_each_cpu(cpu, cpu_online_mask)
+		smp_send_reschedule(cpu);
+
+	return 0;
+}
+
+int dtso_get_memory_consistency_model(void)
+{
+	unsigned long cur_model = get_memory_consistency_model(current);
+
+	if (cur_model == RISCV_MEMORY_CONSISTENCY_MODEL_TSO)
+		return PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO;
+
+	return PR_MEMORY_CONSISTENCY_MODEL_RISCV_WMO;
+}
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 579662731eaa..20264bdc3092 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -308,5 +308,7 @@ struct prctl_mm_map {
 
 #define PR_SET_MEMORY_CONSISTENCY_MODEL		71
 #define PR_GET_MEMORY_CONSISTENCY_MODEL		72
+# define PR_MEMORY_CONSISTENCY_MODEL_RISCV_WMO	1
+# define PR_MEMORY_CONSISTENCY_MODEL_RISCV_TSO	2
 
 #endif /* _LINUX_PRCTL_H */
-- 
2.43.0