with ``.si_code = SEGV_MTEAERR`` and ``.si_addr = 0`` (the faulting
address is unknown).
+- *Asymmetric* - Reads are handled as for synchronous mode while writes
+ are handled as for asynchronous mode.
+
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``
contains any number of the following values in the ``PR_MTE_TCF_MASK``
(ignored if combined with other options)
- ``PR_MTE_TCF_SYNC`` - *Synchronous* tag check fault mode
- ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode
+- ``PR_MTE_TCF_ASYMM`` - *Asymmetric* 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
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``.
+privileged user may write the value ``async``, ``sync`` or ``asymm``. 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 is in the task's set of provided tag checking 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.
+selected by the kernel using the preference order:
+
+ 1. Asynchronous
+ 2. Asymmetric
+ 3. Synchronous
+
+If asymmetric mode is specified by the program but not supported by
+either the system or the kernel then an error will be returned.
Initial process state
---------------------