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authorMark Brown <broonie@kernel.org>2022-02-16 20:32:21 +0300
committerWill Deacon <will@kernel.org>2022-02-25 17:41:05 +0300
commit3f9ab2a6986fe2930782408cc2a6c25e3e5cb98f (patch)
tree41f0a01d44fcf92000a1cac09479e2a05fb4a55d /Documentation/arm64
parent38ddf7dafaeaf3fcdea65b3b4dfb06b4bcd9cc15 (diff)
downloadlinux-3f9ab2a6986fe2930782408cc2a6c25e3e5cb98f.tar.xz
arm64/mte: Document ABI for asymmetric mode
MTE3 adds a new mode which is synchronous for reads but asynchronous for writes. 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> Link: https://lore.kernel.org/r/20220216173224.2342152-2-broonie@kernel.org Signed-off-by: Will Deacon <will@kernel.org>
Diffstat (limited to 'Documentation/arm64')
-rw-r--r--Documentation/arm64/memory-tagging-extension.rst21
1 files changed, 15 insertions, 6 deletions
diff --git a/Documentation/arm64/memory-tagging-extension.rst b/Documentation/arm64/memory-tagging-extension.rst
index 7b99c8f428eb..42677d48a114 100644
--- a/Documentation/arm64/memory-tagging-extension.rst
+++ b/Documentation/arm64/memory-tagging-extension.rst
@@ -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
@@ -139,18 +143,23 @@ 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.
+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
---------------------