SPLITLOCK test: add the README document of split lock test

This document introduces what is split lock and how to test it in user
mode.

Signed-off-by: Tony Zhu <[email protected]>

splitlock/README.md

@@ -0,0 +1,29 @@
+# SPLIT LOCK
+
+## Description
+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.
+
+## Usage
+make
+./sl_test
+1. Linux kernel driver will export the split_lock_detect flag to /proc/cpuinfo
+   if hardware supports this feature.
+2. Test if the locked instruction will trigger #AC. If no split_lock_detect is
+   not set by default or it is split_lock_detect=warn, #AC will be triggered.
+   The boot parameter could also be set as fatal or ratelimit(ratelimit:1 max
+   ratelimit:1000). If split_lock_detect=fatal, split lock test will output
+   "Caught SIGBUS/#AC" in console.
+   For more, please refer to handle_bus_lock in arch/x86/kernel/cpu/intel.c.
+Examples of exception in dmesg:
+x86/split lock detection: #AC: sl_test/4354 took a split_lock trap at address: 0x401231
+x86/split lock detection: #DB: sl_test/5137 took a bus_lock trap at address: 0x4011f5
+
+## Expected result
+All test results should show pass, no fail.