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FROMLIST: arm64/mte: Document ABI for asymmetric mode 

MTE3 adds a new mode which is synchronous for writes but asynchronous for
reads. Document the userspace ABI for this feature, we call the new mode
ASYMM and add a new prctl flag and mte_tcf_preferred value for it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Vincenzo Frascino <Vincenzo.Frascino@arm.com>
Link: https://lore.kernel.org/linux-arm-kernel/20220127195712.748150-2-broonie@kernel.org/
Change-Id: Iad3a5d23b59656072ca5962b2012d74adc1b210b
Bug: 217221156
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
This commit is contained in:
Mark Brown 2022-01-27 19:57:09 +00:00 committed by Andrey Konovalov
parent 81581c794a
commit 926ce98105
1 changed files with 7 additions and 5 deletions
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12
Documentation/arm64/memory-tagging-extension.rst
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@ -76,6 +76,9 @@ configurable behaviours:
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``
@ -85,6 +88,7 @@ bit-field:
(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
@ -139,16 +143,14 @@ 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``.
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.