Documentation/x86: Add ratelimit in buslock.rst

ratelimit is a new command line option for bus lock handling. Add proper
documentation.

[ tglx: Massaged documentation ]

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20210419214958.4035512-5-fenghua.yu@intel.com

diff --git a/Documentation/x86/buslock.rst b/Documentation/x86/buslock.rst
index 159ff6b..7c051e7 100644 (file)
--- a/Documentation/x86/buslock.rst
+++ b/Documentation/x86/buslock.rst
@@ -63,6 +63,11 @@ parameter "split_lock_detect". Here is a summary of different options:
 |                 |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
 ======
@@ -102,3 +107,20 @@ 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.