[RFC PATCH 3/5] uapi: prctl: Add new prctl call to set/get the memory consistency model

[Christoph Muellner]

From: Christoph Müllner <christoph.muellner at vrull.eu>

Some ISAs have a weak default memory consistency model and allow to switch
to a more strict model at runtime. This patch adds calls to the prctl
interface which allow to get and set the current memory consistency
model.

The implementation follows the way other prctl calls are implemented by
disabling them unless arch-specific code provides the relevant macros.

Signed-off-by: Christoph Müllner <christoph.muellner at vrull.eu>
---
 .../mm/dynamic-memory-consistency-model.rst   | 58 +++++++++++++++++++
 include/uapi/linux/prctl.h                    |  3 +
 kernel/sys.c                                  | 12 ++++
 3 files changed, 73 insertions(+)
 create mode 100644 Documentation/mm/dynamic-memory-consistency-model.rst

diff --git a/Documentation/mm/dynamic-memory-consistency-model.rst b/Documentation/mm/dynamic-memory-consistency-model.rst
new file mode 100644
index 000000000000..21675b41ec84
--- /dev/null
+++ b/Documentation/mm/dynamic-memory-consistency-model.rst
@@ -0,0 +1,58 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+Dynamic memory consistency model
+================================
+
+This document gives an overview of the userspace interface to change memory
+consistency model at run-time.
+
+
+What is a memory consistency model?
+===================================
+
+The memory consistency model is a set of guarantees a CPU architecture
+provides about (re-)ordering memory accesses. Each architecture defines
+its own model and set of rules within that, which are carefully specified.
+The provided guarantees have consequences for the microarchitectures (e.g.,
+some memory consistency models allow reordering stores after loads) and
+the software executed within this model (memory consistency models that
+allow reordering memory accesses provide memory barrier instructions
+to enforce additional guarantees when needed explicitly).
+
+Details about the architecture-independent memory consistency model abstraction
+in the Linux kernel and the use of the different types of memory barriers
+can be found here:
+
+	Documentation/memory-barriers.txt
+
+Two models can be in a weaker/stronger relation. I.e., a consistency
+model A is weaker/stronger than another model B if A provides a subset/superset
+of the constraints that B provides.
+
+Some architectures define more than one memory consistency model.
+On such architectures, switching the memory consistency model at run-time
+to a stronger one is possible because software written for the weaker model is
+compatible with the constraints of the stronger model.
+
+If two models are not in a weaker/stronger relation, switching between
+them will violate the consistency assumptions that the software was
+written under (i.e., causing subtle bugs that are very hard to debug).
+
+User API via prctl
+==================
+
+Two prctl calls are defined to get/set the active memory consistency model:
+
+* prctl(PR_GET_MEMORY_CONSISTENCY_MODEL)
+
+    Returns the active memory consistency model for the calling process/thread.
+    If the architecture does not support dynamic memory consistency models,
+    then -1 is returned, and errno is set to EINVAL.
+
+* prctl(PR_SET_MEMORY_CONSISTENCY_MODEL, unsigned long new_model)
+
+    Switches the memory consistency model for the calling process/thread
+    to the given model. If the architecture does not support dynamic
+    memory consistency models or does not support the provided model, then
+    -1 is returned, and errno is set to EINVAL.
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 370ed14b1ae0..579662731eaa 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -306,4 +306,7 @@ struct prctl_mm_map {
 # define PR_RISCV_V_VSTATE_CTRL_NEXT_MASK	0xc
 # define PR_RISCV_V_VSTATE_CTRL_MASK		0x1f
 
+#define PR_SET_MEMORY_CONSISTENCY_MODEL		71
+#define PR_GET_MEMORY_CONSISTENCY_MODEL		72
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index e219fcfa112d..a8a217a10767 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -146,6 +146,12 @@
 #ifndef RISCV_V_GET_CONTROL
 # define RISCV_V_GET_CONTROL()		(-EINVAL)
 #endif
+#ifndef SET_MEMORY_CONSISTENCY_MODEL
+# define SET_MEMORY_CONSISTENCY_MODEL	(-EINVAL)
+#endif
+#ifndef GET_MEMORY_CONSISTENCY_MODEL
+# define GET_MEMORY_CONSISTENCY_MODEL	(-EINVAL)
+#endif
 
 /*
  * this is where the system-wide overflow UID and GID are defined, for
@@ -2743,6 +2749,12 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 	case PR_RISCV_V_GET_CONTROL:
 		error = RISCV_V_GET_CONTROL();
 		break;
+	case PR_SET_MEMORY_CONSISTENCY_MODEL:
+		error = SET_MEMORY_CONSISTENCY_MODEL(arg2);
+		break;
+	case PR_GET_MEMORY_CONSISTENCY_MODEL:
+		error = GET_MEMORY_CONSISTENCY_MODEL();
+		break;
 	default:
 		error = -EINVAL;
 		break;
-- 
2.41.0