linux-kernel - [PATCH] [7/7] Add generic clearcpuid=... option

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Date:	Fri, 18 Jan 2008 18:27:23 +0100 (CET)
From:	Andi Kleen <ak@...e.de>
To:	mingo@...e.hu, hpa@...or.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org
Subject: [PATCH] [7/7] Add generic clearcpuid=... option


Add a generic option to clear any cpuid bit. I added it because it was
very easy to add with the new generic cpuid disable bitmap and perhaps
it will be useful in the future.

Signed-off-by: Andi Kleen <ak@...e.de>

---
 Documentation/kernel-parameters.txt |   13 +++++++++++++
 arch/x86/kernel/cpu/common.c        |   11 +++++++++++
 arch/x86/kernel/setup_64.c          |   11 +++++++++++
 3 files changed, 35 insertions(+)

Index: linux/arch/x86/kernel/cpu/common.c
===================================================================
--- linux.orig/arch/x86/kernel/cpu/common.c
+++ linux/arch/x86/kernel/cpu/common.c
@@ -572,6 +572,17 @@ void __cpuinit print_cpu_info(struct cpu
 		printk("\n");
 }
 
+static __init int setup_disablecpuid(char *arg)
+{
+	int bit;
+	if (get_option(&arg, &bit) && bit < NCAPINTS*32)
+		setup_clear_cpu_cap(bit);
+	else
+		return 0;
+	return 1;
+}
+__setup("clearcpuid=", setup_disablecpuid);
+
 cpumask_t cpu_initialized __cpuinitdata = CPU_MASK_NONE;
 
 /* This is hacky. :)
Index: linux/arch/x86/kernel/setup_64.c
===================================================================
--- linux.orig/arch/x86/kernel/setup_64.c
+++ linux/arch/x86/kernel/setup_64.c
@@ -1060,6 +1060,17 @@ void __cpuinit print_cpu_info(struct cpu
 		printk(KERN_CONT "\n");
 }
 
+static __init int setup_disablecpuid(char *arg)
+{
+	int bit;
+	if (get_option(&arg, &bit) && bit < NCAPINTS*32)
+		setup_clear_cpu_cap(bit);
+	else
+		return 0;
+	return 1;
+}
+__setup("clearcpuid=", setup_disablecpuid);
+
 /*
  *	Get CPU information for use by the procfs.
  */
Index: linux/Documentation/kernel-parameters.txt
===================================================================
--- linux.orig/Documentation/kernel-parameters.txt
+++ linux/Documentation/kernel-parameters.txt
@@ -408,6 +408,19 @@ and is between 256 and 4096 characters. 
 			[SPARC64] tick
 			[X86-64] hpet,tsc
 
+	clearcpuid=BITNUM [X86]
+			Disable CPUID feature X for the kernel. See
+			include/asm-x86/cpufeature.h for the valid bit numbers.
+			Note 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
+			some critical bits.
+
 	code_bytes	[IA32/X86_64] How many bytes of object code to print
 			in an oops report.
 			Range: 0 - 8192
--
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