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>
diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 160b10c..edb19b3 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -640,3 +640,20 @@
This sysfs interface exposes the number of SPURR ticks
for cpuX when it was idle.
+
+What: /sys/devices/system/cpu/cpuX/mte_tcf_preferred
+Date: July 2021
+Contact: Linux ARM Kernel Mailing list <linux-arm-kernel@lists.infradead.org>
+Description: Preferred MTE tag checking mode
+
+ When a user program specifies more than one MTE tag checking
+ mode, this sysfs node is used to specify which mode should
+ be preferred when scheduling a task on that CPU. Possible
+ values:
+
+ ================ ==============================================
+ "sync" Prefer synchronous mode
+ "async" Prefer asynchronous mode
+ ================ ==============================================
+
+ See also: Documentation/arm64/memory-tagging-extension.rst
diff --git a/Documentation/arm64/memory-tagging-extension.rst b/Documentation/arm64/memory-tagging-extension.rst
index b540178..7b99c8f 100644
--- a/Documentation/arm64/memory-tagging-extension.rst
+++ b/Documentation/arm64/memory-tagging-extension.rst
@@ -77,14 +77,20 @@
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``
bit-field:
-- ``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 @@
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 @@
return EXIT_FAILURE;
/*
- * 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.
*/
if (prctl(PR_SET_TAGGED_ADDR_CTRL,
- PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | (0xfffe << PR_MTE_TAG_SHIFT),
+ PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | PR_MTE_TCF_ASYNC |
+ (0xfffe << PR_MTE_TAG_SHIFT),
0, 0, 0)) {
perror("prctl() failed");
return EXIT_FAILURE;