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+===================
+Speculation Control
+===================
+
+Quite some CPUs have speculation-related misfeatures which are in
+fact vulnerabilities causing data leaks in various forms even across
+privilege domains.
+
+The kernel provides mitigation for such vulnerabilities in various
+forms. Some of these mitigations are compile-time configurable and some
+can be supplied on the kernel command line.
+
+There is also a class of mitigations which are very expensive, but they can
+be restricted to a certain set of processes or tasks in controlled
+environments. The mechanism to control these mitigations is via
+:manpage:`prctl(2)`.
+
+There are two prctl options which are related to this:
+
+ * PR_GET_SPECULATION_CTRL
+
+ * PR_SET_SPECULATION_CTRL
+
+PR_GET_SPECULATION_CTRL
+-----------------------
+
+PR_GET_SPECULATION_CTRL returns the state of the speculation misfeature
+which is selected with arg2 of prctl(2). The return value uses bits 0-3 with
+the following meaning:
+
+==== ====================== ==================================================
+Bit Define Description
+==== ====================== ==================================================
+0 PR_SPEC_PRCTL Mitigation can be controlled per task by
+ PR_SET_SPECULATION_CTRL.
+1 PR_SPEC_ENABLE The speculation feature is enabled, mitigation is
+ disabled.
+2 PR_SPEC_DISABLE The speculation feature is disabled, mitigation is
+ enabled.
+3 PR_SPEC_FORCE_DISABLE Same as PR_SPEC_DISABLE, but cannot be undone. A
+ subsequent prctl(..., PR_SPEC_ENABLE) will fail.
+4 PR_SPEC_DISABLE_NOEXEC Same as PR_SPEC_DISABLE, but the state will be
+ cleared on :manpage:`execve(2)`.
+==== ====================== ==================================================
+
+If all bits are 0 the CPU is not affected by the speculation misfeature.
+
+If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is
+available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation
+misfeature will fail.
+
+.. _set_spec_ctrl:
+
+PR_SET_SPECULATION_CTRL
+-----------------------
+
+PR_SET_SPECULATION_CTRL allows to control the speculation misfeature, which
+is selected by arg2 of :manpage:`prctl(2)` per task. arg3 is used to hand
+in the control value, i.e. either PR_SPEC_ENABLE or PR_SPEC_DISABLE or
+PR_SPEC_FORCE_DISABLE.
+
+Common error codes
+------------------
+======= =================================================================
+Value Meaning
+======= =================================================================
+EINVAL The prctl is not implemented by the architecture or unused
+ prctl(2) arguments are not 0.
+
+ENODEV arg2 is selecting a not supported speculation misfeature.
+======= =================================================================
+
+PR_SET_SPECULATION_CTRL error codes
+-----------------------------------
+======= =================================================================
+Value Meaning
+======= =================================================================
+0 Success
+
+ERANGE arg3 is incorrect, i.e. it's neither PR_SPEC_ENABLE nor
+ PR_SPEC_DISABLE nor PR_SPEC_FORCE_DISABLE.
+
+ENXIO Control of the selected speculation misfeature is not possible.
+ See PR_GET_SPECULATION_CTRL.
+
+EPERM Speculation was disabled with PR_SPEC_FORCE_DISABLE and caller
+ tried to enable it again.
+======= =================================================================
+
+Speculation misfeature controls
+-------------------------------
+- PR_SPEC_STORE_BYPASS: Speculative Store Bypass
+
+ Invocations:
+ * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, 0, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_DISABLE_NOEXEC, 0, 0);
+
+- PR_SPEC_INDIR_BRANCH: Indirect Branch Speculation in User Processes
+ (Mitigate Spectre V2 style attacks against user processes)
+
+ Invocations:
+ * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, 0, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0);
+ * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);