[RFC PATCH v2 1/6] mm: Add dynamic memory consistency model switching

[Christoph Müllner]

Some architectures have support to change the memory consistency model
at run time. This patch adds a new field 'active_memory_consistency_model'
to task_struct that allows architecture code to store the active model
as a per-process property.

To avoid useless overhead, the mechanism needs to be explicitly
enabled in the architecture's Kconfig.

Signed-off-by: Christoph Müllner <christoph.muellner at vrull.eu>
---
 .../mm/dynamic-memory-consistency-model.rst   | 49 +++++++++++++++++++
 Documentation/mm/index.rst                    |  1 +
 arch/Kconfig                                  | 14 ++++++
 include/linux/sched.h                         |  5 ++
 4 files changed, 69 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..3117c3d82b2b
--- /dev/null
+++ b/Documentation/mm/dynamic-memory-consistency-model.rst
@@ -0,0 +1,49 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================================
+Dynamic memory consistency model switching
+==========================================
+
+:Author: Christoph Müllner <christoph.muellner at vrull.eu>
+:Date: 1 Feb 2024
+
+This document gives an overview about dynamic memory consistency model
+switching for user mode at run-time.
+
+Memory consistency models
+=========================
+
+A 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).
+
+The following restrictions apply for switching the memory consistency model
+at run-time:
+
+* Only switching from a weaker to a stronger model is safe.
+* The stronger memory model affects all threads of a process, when running in user mode.
+* Forked processes derive their active memory model from their parents.
diff --git a/Documentation/mm/index.rst b/Documentation/mm/index.rst
index 31d2ac306438..36d40502b421 100644
--- a/Documentation/mm/index.rst
+++ b/Documentation/mm/index.rst
@@ -43,6 +43,7 @@ above structured documentation, or deleted if it has served its purpose.
    arch_pgtable_helpers
    balance
    damon/index
+   dynamic-memory-consistency-model
    free_page_reporting
    hmm
    hwpoison
diff --git a/arch/Kconfig b/arch/Kconfig
index a5af0edd3eb8..89d4e27f9b80 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1479,6 +1479,20 @@ config ARCH_HAS_NONLEAF_PMD_YOUNG
 	  address translations. Page table walkers that clear the accessed bit
 	  may use this capability to reduce their search space.
 
+config ARCH_HAS_DYNAMIC_MEMORY_CONSISTENCY_MODEL
+	bool
+	help
+	  An arch should select this symbol if it supports switching
+	  the memory consistency model at run-time.
+
+config DYNAMIC_MEMORY_CONSISTENCY_MODEL
+	bool "Dynamic memory consistency model support"
+	depends on ARCH_HAS_DYNAMIC_MEMORY_CONSISTENCY_MODEL
+	default y
+	help
+	  This option turns on the support to switch the memory consistency
+	  model at runtime on a per-process-base.
+
 source "kernel/gcov/Kconfig"
 
 source "scripts/gcc-plugins/Kconfig"
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ffe8f618ab86..5cbd3a3b80ab 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -979,6 +979,11 @@ struct task_struct {
 	/* Canary value for the -fstack-protector GCC feature: */
 	unsigned long			stack_canary;
 #endif
+
+#ifdef CONFIG_DYNAMIC_MEMORY_CONSISTENCY_MODEL
+	unsigned long			memory_consistency_model;
+#endif
+
 	/*
 	 * Pointers to the (original) parent process, youngest child, younger sibling,
 	 * older sibling, respectively.  (p->father can be replaced with
-- 
2.43.0