path: root/Documentation/arm64
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authorPeter Collingbourne <pcc@google.com>2021-07-27 13:52:59 -0700
committerCatalin Marinas <catalin.marinas@arm.com>2021-07-28 18:39:26 +0100
commit80c7c36fb3ddea8e06f75822bfb7634f64d0edcb (patch)
treecd67067a9cdc3dc8211aaf0238834abf37925613 /Documentation/arm64
parentarm64: mte: introduce a per-CPU tag checking mode preference (diff)
Documentation: document the preferred tag checking mode feature
Document the functionality added in the previous patches. Link: https://linux-review.googlesource.com/id/I48217cc3e8b8da33abc08cbaddc11cf4360a1b86 Signed-off-by: Peter Collingbourne <pcc@google.com> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Acked-by: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20210727205300.2554659-6-pcc@google.com Acked-by: Will Deacon <will@kernel.org> [catalin.marinas@arm.com: clarify that the change happens on task scheduling] Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Diffstat (limited to 'Documentation/arm64')
1 files changed, 41 insertions, 7 deletions
diff --git a/Documentation/arm64/memory-tagging-extension.rst b/Documentation/arm64/memory-tagging-extension.rst
index b540178a93f8..7b99c8f428eb 100644
--- a/Documentation/arm64/memory-tagging-extension.rst
+++ b/Documentation/arm64/memory-tagging-extension.rst
@@ -77,14 +77,20 @@ configurable behaviours:
address is unknown).
The user can select the above modes, per thread, using the
-``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where
-``flags`` contain one of the following values in the ``PR_MTE_TCF_MASK``
+``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where ``flags``
+contains any number of the following values in the ``PR_MTE_TCF_MASK``
-- ``PR_MTE_TCF_NONE`` - *Ignore* tag check faults
+- ``PR_MTE_TCF_NONE``  - *Ignore* tag check faults
+ (ignored if combined with other options)
- ``PR_MTE_TCF_SYNC`` - *Synchronous* tag check fault mode
- ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode
+If no modes are specified, tag check faults are ignored. If a single
+mode is specified, the program will run in that mode. If multiple
+modes are specified, the mode is selected as described in the "Per-CPU
+preferred tag checking modes" section below.
The current tag check fault mode can be read using the
``prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)`` system call.
@@ -120,13 +126,39 @@ in the ``PR_MTE_TAG_MASK`` bit-field.
interface provides an include mask. An include mask of ``0`` (exclusion
mask ``0xffff``) results in the CPU always generating tag ``0``.
+Per-CPU preferred tag checking mode
+On some CPUs the performance of MTE in stricter tag checking modes
+is similar to that of less strict tag checking modes. This makes it
+worthwhile to enable stricter checks on those CPUs when a less strict
+checking mode is requested, in order to gain the error detection
+benefits of the stricter checks without the performance downsides. To
+support this scenario, a privileged user may configure a stricter
+tag checking mode as the CPU's preferred tag checking mode.
+The preferred tag checking mode for each CPU is controlled by
+``/sys/devices/system/cpu/cpu<N>/mte_tcf_preferred``, to which a
+privileged user may write the value ``async`` or ``sync``. The default
+preferred mode for each CPU is ``async``.
+To allow a program to potentially run in the CPU's preferred tag
+checking mode, the user program may set multiple tag check fault mode
+bits in the ``flags`` argument to the ``prctl(PR_SET_TAGGED_ADDR_CTRL,
+flags, 0, 0, 0)`` system call. If the CPU's preferred tag checking
+mode is in the task's set of provided tag checking modes (this will
+always be the case at present because the kernel only supports two
+tag checking modes, but future kernels may support more modes), that
+mode will be selected. Otherwise, one of the modes in the task's mode
+set will be selected in a currently unspecified manner.
Initial process state
On ``execve()``, the new process has the following configuration:
- ``PR_TAGGED_ADDR_ENABLE`` set to 0 (disabled)
-- Tag checking mode set to ``PR_MTE_TCF_NONE``
+- No tag checking modes are selected (tag check faults ignored)
- ``PR_MTE_TAG_MASK`` set to 0 (all tags excluded)
- ``PSTATE.TCO`` set to 0
- ``PROT_MTE`` not set on any of the initial memory maps
@@ -251,11 +283,13 @@ Example of correct usage
- * Enable the tagged address ABI, synchronous MTE tag check faults and
- * allow all non-zero tags in the randomly generated set.
+ * Enable the tagged address ABI, synchronous or asynchronous MTE
+ * tag check faults (based on per-CPU preference) and allow all
+ * non-zero tags in the randomly generated set.
+ (0xfffe << PR_MTE_TAG_SHIFT),
0, 0, 0)) {
perror("prctl() failed");