127 lines
4.6 KiB
ReStructuredText
127 lines
4.6 KiB
ReStructuredText
.. 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 and |and continues to run. |
|
|
| |disable future checking | |
|
|
| |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.
|