Merge tag 'x86-urgent-2022-08-13' of git://git.kernel.org/pub/scm/linux/kernel/git...

[platform/kernel/linux-rpi.git] / Documentation / admin-guide / kernel-parameters.txt

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index db5de5f..d7f3090 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5274,20 +5274,33 @@
                        Speculative Code Execution with Return Instructions)
                        vulnerability.
 
+                       AMD-based UNRET and IBPB mitigations alone do not stop
+                       sibling threads from influencing the predictions of other
+                       sibling threads. For that reason, STIBP is used on pro-
+                       cessors that support it, and mitigate SMT on processors
+                       that don't.
+
                        off          - no mitigation
                        auto         - automatically select a migitation
                        auto,nosmt   - automatically select a mitigation,
                                       disabling SMT if necessary for
                                       the full mitigation (only on Zen1
                                       and older without STIBP).
-                       ibpb         - mitigate short speculation windows on
-                                      basic block boundaries too. Safe, highest
-                                      perf impact.
-                       unret        - force enable untrained return thunks,
-                                      only effective on AMD f15h-f17h
-                                      based systems.
-                       unret,nosmt  - like unret, will disable SMT when STIBP
-                                      is not available.
+                       ibpb         - On AMD, mitigate short speculation
+                                      windows on basic block boundaries too.
+                                      Safe, highest perf impact. It also
+                                      enables STIBP if present. Not suitable
+                                      on Intel.
+                       ibpb,nosmt   - Like "ibpb" above but will disable SMT
+                                      when STIBP is not available. This is
+                                      the alternative for systems which do not
+                                      have STIBP.
+                       unret        - Force enable untrained return thunks,
+                                      only effective on AMD f15h-f17h based
+                                      systems.
+                       unret,nosmt  - Like unret, but will disable SMT when STIBP
+                                      is not available. This is the alternative for
+                                      systems which do not have STIBP.
 
                        Selecting 'auto' will choose a mitigation method at run
                        time according to the CPU.