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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>
This commit is contained in:
Peter Collingbourne 2021-07-27 13:52:59 -07:00 committed by Catalin Marinas
parent dd061616ed
commit 80c7c36fb3
2 changed files with 58 additions and 7 deletions
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17
Documentation/ABI/testing/sysfs-devices-system-cpu
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@ -640,3 +640,20 @@ Description: SPURR ticks for cpuX when it was idle.
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

48
Documentation/arm64/memory-tagging-extension.rst
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@ -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``
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 @@ 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
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;