x86/bugs: Remove duplicate Spectre cmdline option descriptions

Duplicating the documentation of all the Spectre kernel cmdline options
in two separate files is unwieldy and error-prone.  Instead just add a
reference to kernel-parameters.txt from spectre.rst.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Link: https://lore.kernel.org/r/450b5f4ffe891a8cc9736ec52b0c6f225bab3f4b.1719381528.git.jpoimboe@kernel.org

Author: jpoimboe

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

@@ -592,85 +592,19 @@ Spectre variant 2
 Mitigation control on the kernel command line
 ---------------------------------------------
 
-Spectre variant 2 mitigation can be disabled or force enabled at the
-kernel command line.
+In general the kernel selects reasonable default mitigations for the
+current CPU.
 
-	nospectre_v1
+Spectre default mitigations can be disabled or changed at the kernel
+command line with the following options:
 
-		[X86,PPC] Disable mitigations for Spectre Variant 1
-		(bounds check bypass). With this option data leaks are
-		possible in the system.
+	- nospectre_v1
+	- nospectre_v2
+	- spectre_v2={option}
+	- spectre_v2_user={option}
+	- spectre_bhi={option}
 
-	nospectre_v2
-
-		[X86] Disable all mitigations for the Spectre variant 2
-		(indirect branch prediction) vulnerability. System may
-		allow data leaks with this option, which is equivalent
-		to spectre_v2=off.
-
-
-        spectre_v2=
-
-		[X86] Control mitigation of Spectre variant 2
-		(indirect branch speculation) vulnerability.
-		The default operation protects the kernel from
-		user space attacks.
-
-		on
-			unconditionally enable, implies
-			spectre_v2_user=on
-		off
-			unconditionally disable, implies
-		        spectre_v2_user=off
-		auto
-			kernel detects whether your CPU model is
-		        vulnerable
-
-		Selecting 'on' will, and 'auto' may, choose a
-		mitigation method at run time according to the
-		CPU, the available microcode, the setting of the
-		CONFIG_MITIGATION_RETPOLINE configuration option,
-		and the compiler with which the kernel was built.
-
-		Selecting 'on' will also enable the mitigation
-		against user space to user space task attacks.
-
-		Selecting 'off' will disable both the kernel and
-		the user space protections.
-
-		Specific mitigations can also be selected manually:
-
-                retpoline               auto pick between generic,lfence
-                retpoline,generic       Retpolines
-                retpoline,lfence        LFENCE; indirect branch
-                retpoline,amd           alias for retpoline,lfence
-                eibrs                   Enhanced/Auto IBRS
-                eibrs,retpoline         Enhanced/Auto IBRS + Retpolines
-                eibrs,lfence            Enhanced/Auto IBRS + LFENCE
-                ibrs                    use IBRS to protect kernel
-
-		Not specifying this option is equivalent to
-		spectre_v2=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.
-
-	spectre_bhi=
-
-		[X86] Control mitigation of Branch History Injection
-		(BHI) vulnerability.  This setting affects the deployment
-		of the HW BHI control and the SW BHB clearing sequence.
-
-		on
-			(default) Enable the HW or SW mitigation as
-			needed.
-		off
-			Disable the mitigation.
-
-For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
+For more details on the available options, refer to Documentation/admin-guide/kernel-parameters.txt
 
 Mitigation selection guide
 --------------------------