+++ /dev/null
-.. SPDX-License-Identifier: GPL-2.0
-
-.. include:: <isonum.txt>
-
-===============================
-Bus lock detection and handling
-===============================
-
-:Copyright: |copy| 2021 Intel Corporation
-:Authors: - Fenghua Yu <fenghua.yu@intel.com>
- - Tony Luck <tony.luck@intel.com>
-
-Problem
-=======
-
-A split lock is any atomic operation whose operand crosses two cache lines.
-Since the operand spans two cache lines and the operation must be atomic,
-the system locks the bus while the CPU accesses the two cache lines.
-
-A bus lock is acquired through either split locked access to writeback (WB)
-memory or any locked access to non-WB memory. This is typically thousands of
-cycles slower than an atomic operation within a cache line. It also disrupts
-performance on other cores and brings the whole system to its knees.
-
-Detection
-=========
-
-Intel processors may support either or both of the following hardware
-mechanisms to detect split locks and bus locks.
-
-#AC exception for split lock detection
---------------------------------------
-
-Beginning with the Tremont Atom CPU split lock operations may raise an
-Alignment Check (#AC) exception when a split lock operation is attemped.
-
-#DB exception for bus lock detection
-------------------------------------
-
-Some CPUs have the ability to notify the kernel by an #DB trap after a user
-instruction acquires a bus lock and is executed. This allows the kernel to
-terminate the application or to enforce throttling.
-
-Software handling
-=================
-
-The kernel #AC and #DB handlers handle bus lock based on the kernel
-parameter "split_lock_detect". Here is a summary of different options:
-
-+------------------+----------------------------+-----------------------+
-|split_lock_detect=|#AC for split lock |#DB for bus lock |
-+------------------+----------------------------+-----------------------+
-|off |Do nothing |Do nothing |
-+------------------+----------------------------+-----------------------+
-|warn |Kernel OOPs |Warn once per task and |
-|(default) |Warn once per task, add a |and continues to run. |
-| |delay, add synchronization | |
-| |to prevent more than one | |
-| |core from executing a | |
-| |split lock in parallel. | |
-| |sysctl split_lock_mitigate | |
-| |can be used to avoid the | |
-| |delay and synchronization | |
-| |When both features are | |
-| |supported, warn in #AC | |
-+------------------+----------------------------+-----------------------+
-|fatal |Kernel OOPs |Send SIGBUS to user. |
-| |Send SIGBUS to user | |
-| |When both features are | |
-| |supported, fatal in #AC | |
-+------------------+----------------------------+-----------------------+
-|ratelimit:N |Do nothing |Limit bus lock rate to |
-|(0 < N <= 1000) | |N bus locks per second |
-| | |system wide and warn on|
-| | |bus locks. |
-+------------------+----------------------------+-----------------------+
-
-Usages
-======
-
-Detecting and handling bus lock may find usages in various areas:
-
-It is critical for real time system designers who build consolidated real
-time systems. These systems run hard real time code on some cores and run
-"untrusted" user processes on other cores. The hard real time cannot afford
-to have any bus lock from the untrusted processes to hurt real time
-performance. To date the designers have been unable to deploy these
-solutions as they have no way to prevent the "untrusted" user code from
-generating split lock and bus lock to block the hard real time code to
-access memory during bus locking.
-
-It's also useful for general computing to prevent guests or user
-applications from slowing down the overall system by executing instructions
-with bus lock.
-
-
-Guidance
-========
-off
----
-
-Disable checking for split lock and bus lock. This option can be useful if
-there are legacy applications that trigger these events at a low rate so
-that mitigation is not needed.
-
-warn
-----
-
-A warning is emitted when a bus lock is detected which allows to identify
-the offending application. This is the default behavior.
-
-fatal
------
-
-In this case, the bus lock is not tolerated and the process is killed.
-
-ratelimit
----------
-
-A system wide bus lock rate limit N is specified where 0 < N <= 1000. This
-allows a bus lock rate up to N bus locks per second. When the bus lock rate
-is exceeded then any task which is caught via the buslock #DB exception is
-throttled by enforced sleeps until the rate goes under the limit again.
-
-This is an effective mitigation in cases where a minimal impact can be
-tolerated, but an eventual Denial of Service attack has to be prevented. It
-allows to identify the offending processes and analyze whether they are
-malicious or just badly written.
-
-Selecting a rate limit of 1000 allows the bus to be locked for up to about
-seven million cycles each second (assuming 7000 cycles for each bus
-lock). On a 2 GHz processor that would be about 0.35% system slowdown.
projects / linux-2.6-block.git / blobdiff