Documentation/arch/x86: Hide clearcpuid=

This option was never meant to be used in production because it solely
clears the X86_FEATURE kernel-internal representation of what CPUID bits
it has detected and doesn't do any *proper* feature disablement like
clearing CR4.CET in the user shadow stack case, for example.

So remove its documentation so that it doesn't get used in production
and people get silly ideas. It is meant strictly for debugging; and if
a chicken bit for properly disabling a feature is warranted, then that
would need proper enablement.

No functional changes.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Mathias Krause <minipli@grsecurity.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://patch.msgid.link/20260520202508.160112-1-bp@kernel.org

Author: bp3tk0v

Documentation/admin-guide/kernel-parameters.txt

@@ -789,24 +789,6 @@ Kernel parameters
 	cio_ignore=	[S390]
 			See Documentation/arch/s390/common_io.rst for details.
 
-	clearcpuid=X[,X...] [X86]
-			Disable CPUID feature X for the kernel. See
-			arch/x86/include/asm/cpufeatures.h for the valid bit
-			numbers X. Note the Linux-specific bits are not necessarily
-			stable over kernel options, but the vendor-specific
-			ones should be.
-			X can also be a string as appearing in the flags: line
-			in /proc/cpuinfo which does not have the above
-			instability issue. However, not all features have names
-			in /proc/cpuinfo.
-			Note that using this option will taint your kernel.
-			Also note that user programs calling CPUID directly
-			or using the feature without checking anything
-			will still see it. This just prevents it from
-			being used by the kernel or shown in /proc/cpuinfo.
-			Also note the kernel might malfunction if you disable
-			some critical bits.
-
 	clk_ignore_unused
 			[CLK]
 			Prevents the clock framework from automatically gating

Documentation/arch/x86/cpuinfo.rst

@@ -187,6 +187,10 @@ to disable features using the feature number as defined in
 Protection can be disabled using clearcpuid=514. The number 514 is calculated
 from #define X86_FEATURE_UMIP (16*32 + 2).
 
+DO NOT USE this cmdline option in production - it is meant to be used only as
+a quick'n'dirty debugging aid to rule out a feature-enabling code is the
+culprit. If you use it, it'll taint the kernel.
+
 In addition, there exists a variety of custom command-line parameters that
 disable specific features. The list of parameters includes, but is not limited
 to, nofsgsbase, nosgx, noxsave, etc. 5-level paging can also be disabled using