linux/Documentation/x86/buslock.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3.. include:: <isonum.txt>
   4
   5===============================
   6Bus lock detection and handling
   7===============================
   8
   9:Copyright: |copy| 2021 Intel Corporation
  10:Authors: - Fenghua Yu <fenghua.yu@intel.com>
  11          - Tony Luck <tony.luck@intel.com>
  12
  13Problem
  14=======
  15
  16A split lock is any atomic operation whose operand crosses two cache lines.
  17Since the operand spans two cache lines and the operation must be atomic,
  18the system locks the bus while the CPU accesses the two cache lines.
  19
  20A bus lock is acquired through either split locked access to writeback (WB)
  21memory or any locked access to non-WB memory. This is typically thousands of
  22cycles slower than an atomic operation within a cache line. It also disrupts
  23performance on other cores and brings the whole system to its knees.
  24
  25Detection
  26=========
  27
  28Intel processors may support either or both of the following hardware
  29mechanisms to detect split locks and bus locks.
  30
  31#AC exception for split lock detection
  32--------------------------------------
  33
  34Beginning with the Tremont Atom CPU split lock operations may raise an
  35Alignment Check (#AC) exception when a split lock operation is attemped.
  36
  37#DB exception for bus lock detection
  38------------------------------------
  39
  40Some CPUs have the ability to notify the kernel by an #DB trap after a user
  41instruction acquires a bus lock and is executed. This allows the kernel to
  42terminate the application or to enforce throttling.
  43
  44Software handling
  45=================
  46
  47The kernel #AC and #DB handlers handle bus lock based on the kernel
  48parameter "split_lock_detect". Here is a summary of different options:
  49
  50+------------------+----------------------------+-----------------------+
  51|split_lock_detect=|#AC for split lock          |#DB for bus lock       |
  52+------------------+----------------------------+-----------------------+
  53|off               |Do nothing                  |Do nothing             |
  54+------------------+----------------------------+-----------------------+
  55|warn              |Kernel OOPs                 |Warn once per task and |
  56|(default)         |Warn once per task and      |and continues to run.  |
  57|                  |disable future checking     |                       |
  58|                  |When both features are      |                       |
  59|                  |supported, warn in #AC      |                       |
  60+------------------+----------------------------+-----------------------+
  61|fatal             |Kernel OOPs                 |Send SIGBUS to user.   |
  62|                  |Send SIGBUS to user         |                       |
  63|                  |When both features are      |                       |
  64|                  |supported, fatal in #AC     |                       |
  65+------------------+----------------------------+-----------------------+
  66|ratelimit:N       |Do nothing                  |Limit bus lock rate to |
  67|(0 < N <= 1000)   |                            |N bus locks per second |
  68|                  |                            |system wide and warn on|
  69|                  |                            |bus locks.             |
  70+------------------+----------------------------+-----------------------+
  71
  72Usages
  73======
  74
  75Detecting and handling bus lock may find usages in various areas:
  76
  77It is critical for real time system designers who build consolidated real
  78time systems. These systems run hard real time code on some cores and run
  79"untrusted" user processes on other cores. The hard real time cannot afford
  80to have any bus lock from the untrusted processes to hurt real time
  81performance. To date the designers have been unable to deploy these
  82solutions as they have no way to prevent the "untrusted" user code from
  83generating split lock and bus lock to block the hard real time code to
  84access memory during bus locking.
  85
  86It's also useful for general computing to prevent guests or user
  87applications from slowing down the overall system by executing instructions
  88with bus lock.
  89
  90
  91Guidance
  92========
  93off
  94---
  95
  96Disable checking for split lock and bus lock. This option can be useful if
  97there are legacy applications that trigger these events at a low rate so
  98that mitigation is not needed.
  99
 100warn
 101----
 102
 103A warning is emitted when a bus lock is detected which allows to identify
 104the offending application. This is the default behavior.
 105
 106fatal
 107-----
 108
 109In this case, the bus lock is not tolerated and the process is killed.
 110
 111ratelimit
 112---------
 113
 114A system wide bus lock rate limit N is specified where 0 < N <= 1000. This
 115allows a bus lock rate up to N bus locks per second. When the bus lock rate
 116is exceeded then any task which is caught via the buslock #DB exception is
 117throttled by enforced sleeps until the rate goes under the limit again.
 118
 119This is an effective mitigation in cases where a minimal impact can be
 120tolerated, but an eventual Denial of Service attack has to be prevented. It
 121allows to identify the offending processes and analyze whether they are
 122malicious or just badly written.
 123
 124Selecting a rate limit of 1000 allows the bus to be locked for up to about
 125seven million cycles each second (assuming 7000 cycles for each bus
 126lock). On a 2 GHz processor that would be about 0.35% system slowdown.
 127