From 1897907cca5aa22cdfcdb7fb8f0644a6add0877d Mon Sep 17 00:00:00 2001 From: Fenghua Yu Date: Mon, 19 Apr 2021 21:49:55 +0000 Subject: [PATCH] Documentation/x86: Add buslock.rst Add buslock.rst to explain bus lock problem and how to detect and handle it. [ tglx: Included it into index.rst and added the missing include ... ] Signed-off-by: Fenghua Yu Signed-off-by: Thomas Gleixner Reviewed-by: Tony Luck Link: https://lore.kernel.org/r/20210419214958.4035512-2-fenghua.yu@intel.com --- Documentation/x86/buslock.rst | 104 ++++++++++++++++++++++++++++++++++ Documentation/x86/index.rst | 1 + 2 files changed, 105 insertions(+) create mode 100644 Documentation/x86/buslock.rst diff --git a/Documentation/x86/buslock.rst b/Documentation/x86/buslock.rst new file mode 100644 index 000000000000..159ff6ba830e --- /dev/null +++ b/Documentation/x86/buslock.rst @@ -0,0 +1,104 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. include:: + +=============================== +Bus lock detection and handling +=============================== + +:Copyright: |copy| 2021 Intel Corporation +:Authors: - Fenghua Yu + - Tony Luck + +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 | | ++------------------+----------------------------+-----------------------+ + +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. diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst index 4693e192b447..0004f5d2283e 100644 --- a/Documentation/x86/index.rst +++ b/Documentation/x86/index.rst @@ -29,6 +29,7 @@ x86-specific Documentation microcode resctrl tsx_async_abort + buslock usb-legacy-support i386/index x86_64/index