[tip: x86/splitlock] Documentation/x86: Add buslock.rst

From: tip-bot2 for Fenghua Yu
Date: Tue May 18 2021 - 10:44:49 EST

The following commit has been merged into the x86/splitlock branch of tip:

Commit-ID: 1897907cca5aa22cdfcdb7fb8f0644a6add0877d
Gitweb: https://git.kernel.org/tip/1897907cca5aa22cdfcdb7fb8f0644a6add0877d
Author: Fenghua Yu <fenghua.yu@xxxxxxxxx>
AuthorDate: Mon, 19 Apr 2021 21:49:55
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 18 May 2021 16:39:31 +02:00

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 <fenghua.yu@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Link: https://lore.kernel.org/r/20210419214958.4035512-2-fenghua.yu@xxxxxxxxx

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 0000000..159ff6b
--- /dev/null
+++ b/Documentation/x86/buslock.rst
@@ -0,0 +1,104 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+Bus lock detection and handling
+:Copyright: |copy| 2021 Intel Corporation
+:Authors: - Fenghua Yu <fenghua.yu@xxxxxxxxx>
+ - Tony Luck <tony.luck@xxxxxxxxx>
+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.
+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 | |
+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.
+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.
+A warning is emitted when a bus lock is detected which allows to identify
+the offending application. This is the default behavior.
+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 4693e19..0004f5d 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -29,6 +29,7 @@ x86-specific Documentation
+ buslock