Merge tag 'x86-splitlock-2021-04-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 bus lock detection updates from Thomas Gleixner:
 "Support for enhanced split lock detection:

  Newer CPUs provide a second mechanism to detect operations with lock
  prefix which go accross a cache line boundary. Such operations have to
  take bus lock which causes a system wide performance degradation when
  these operations happen frequently.

  The new mechanism is not using the #AC exception. It triggers #DB and
  is restricted to operations in user space. Kernel side split lock
  access can only be detected by the #AC based variant.

  Contrary to the #AC based mechanism the #DB based variant triggers
  _after_ the instruction was executed. The mechanism is CPUID
  enumerated and contrary to the #AC version which is based on the magic
  TEST_CTRL_MSR and model/family based enumeration on the way to become
  architectural"

* tag 'x86-splitlock-2021-04-26' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  Documentation/admin-guide: Change doc for split_lock_detect parameter
  x86/traps: Handle #DB for bus lock
  x86/cpufeatures: Enumerate #DB for bus lock detection

1 files changed, 16 insertions, 6 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index dd5947a96714..0136cdf7e55d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5111,27 +5111,37 @@
 	spia_peddr=
 
 	split_lock_detect=
-			[X86] Enable split lock detection
+			[X86] Enable split lock detection or bus lock detection
 
 			When enabled (and if hardware support is present), atomic
 			instructions that access data across cache line
-			boundaries will result in an alignment check exception.
+			boundaries will result in an alignment check exception
+			for split lock detection or a debug exception for
+			bus lock detection.
 
 			off	- not enabled
 
-			warn	- the kernel will emit rate limited warnings
+			warn	- the kernel will emit rate-limited warnings
 				  about applications triggering the #AC
-				  exception. This mode is the default on CPUs
-				  that supports split lock detection.
+				  exception or the #DB exception. This mode is
+				  the default on CPUs that support split lock
+				  detection or bus lock detection. Default
+				  behavior is by #AC if both features are
+				  enabled in hardware.
 
 			fatal	- the kernel will send SIGBUS to applications
-				  that trigger the #AC exception.
+				  that trigger the #AC exception or the #DB
+				  exception. Default behavior is by #AC if
+				  both features are enabled in hardware.
 
 			If an #AC exception is hit in the kernel or in
 			firmware (i.e. not while executing in user mode)
 			the kernel will oops in either "warn" or "fatal"
 			mode.
 
+			#DB exception for bus lock is triggered only when
+			CPL > 0.
+
 	srbds=		[X86,INTEL]
 			Control the Special Register Buffer Data Sampling
 			(SRBDS) mitigation.