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@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@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