linux-kernel - [PATCH v7 21/21] x86/clearcpuid: Change document for kernel option clearcpuid

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Date:   Wed, 17 Apr 2019 14:34:11 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        "H Peter Anvin" <hpa@...or.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Ashok Raj" <ashok.raj@...el.com>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Ravi V Shankar" <ravi.v.shankar@...el.com>,
        "Xiaoyao Li " <xiaoyao.li@...el.com>,
        "Christopherson Sean J" <sean.j.christopherson@...el.com>,
        "Kalle Valo" <kvalo@...eaurora.org>,
        "Michael Chan" <michael.chan@...adcom.com>
Cc:     "linux-kernel" <linux-kernel@...r.kernel.org>,
        "x86" <x86@...nel.org>, kvm@...r.kernel.org,
        netdev@...r.kernel.org, linux-wireless@...r.kernel.org,
        Fenghua Yu <fenghua.yu@...el.com>
Subject: [PATCH v7 21/21] x86/clearcpuid: Change document for kernel option clearcpuid

Since kernel option clearcpuid now supports multiple options and CPU
capability flags, the document needs to be changed.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
---
 .../admin-guide/kernel-parameters.txt          | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2b8ee90bb644..0cbeda6d7f16 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -563,17 +563,21 @@
 			loops can be debugged more effectively on production
 			systems.
 
-	clearcpuid=BITNUM [X86]
+	clearcpuid=BITNUM | FLAG [X86]
 			Disable CPUID feature X for the kernel. See
 			arch/x86/include/asm/cpufeatures.h for the valid bit
-			numbers. Note the Linux specific bits are not necessarily
+			numbers or /proc/cpuinfo for valid CPU flags.
+			Multiple options can be used to disable a few features.
+			This prevents the feature from being used by the
+			kernel or shown in /proc/cpuinfo or shown in CPUID
+			called directly by user programs.
+			A few notes:
+			- The Linux specific bits are not necessarily
 			stable over kernel options, but the vendor specific
 			ones should be.
-			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
+			- User programs using the feature without checking
+			anything will still use it.
+			- The kernel might malfunction if you disable
 			some critical bits.
 
 	cma=nn[MG]@[start[MG][-end[MG]]]
-- 
2.19.1