linux-kernel - Re: [RFC PATCH] Use feature bit names in clearcpuid=

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Message-ID: <20200920185914.GD7473@zn.tnic>
Date:   Sun, 20 Sep 2020 20:59:14 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] Use feature bit names in clearcpuid=

On Sun, Sep 20, 2020 at 02:35:27PM -0400, Arvind Sankar wrote:
> Thanks. Maybe also mention in the documentation that names can now be
> used.

Already did it as an incremental ontop. Will merge later:

---
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ffe864390c5a..49988e891d7f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -577,12 +577,17 @@
 			loops can be debugged more effectively on production
 			systems.
 
-	clearcpuid=BITNUM[,BITNUM...] [X86]
+	clearcpuid=X[,X...] [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
-			stable over kernel options, but the vendor specific
+			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


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette