x86: deduplicate the spectre_v2_user documentation

This would need updating to make prctl be the new default, but it's
simpler to delete it and refer to the dup.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20201105001406.13005-2-aarcange@redhat.com

Author: aagit

Documentation/admin-guide/hw-vuln/spectre.rst

@@ -593,61 +593,14 @@ kernel command line.
 		Not specifying this option is equivalent to
 		spectre_v2=auto.
 
-For user space mitigation:
-
-        spectre_v2_user=
-
-		[X86] Control mitigation of Spectre variant 2
-		(indirect branch speculation) vulnerability between
-		user space tasks
-
-		on
-			Unconditionally enable mitigations. Is
-			enforced by spectre_v2=on
-
-		off
-			Unconditionally disable mitigations. Is
-			enforced by spectre_v2=off
-
-		prctl
-			Indirect branch speculation is enabled,
-			but mitigation can be enabled via prctl
-			per thread. The mitigation control state
-			is inherited on fork.
-
-		prctl,ibpb
-			Like "prctl" above, but only STIBP is
-			controlled per thread. IBPB is issued
-			always when switching between different user
-			space processes.
-
-		seccomp
-			Same as "prctl" above, but all seccomp
-			threads will enable the mitigation unless
-			they explicitly opt out.
-
-		seccomp,ibpb
-			Like "seccomp" above, but only STIBP is
-			controlled per thread. IBPB is issued
-			always when switching between different
-			user space processes.
-
-		auto
-			Kernel selects the mitigation depending on
-			the available CPU features and vulnerability.
-
-		Default mitigation:
-		If CONFIG_SECCOMP=y then "seccomp", otherwise "prctl"
-
-		Not specifying this option is equivalent to
-		spectre_v2_user=auto.
-
 		In general the kernel by default selects
 		reasonable mitigations for the current CPU. To
 		disable Spectre variant 2 mitigations, boot with
 		spectre_v2=off. Spectre variant 1 mitigations
 		cannot be disabled.
 
+For spectre_v2_user see :doc:`/admin-guide/kernel-parameters`.
+
 Mitigation selection guide
 --------------------------