x86: 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@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/kernel-parameters.txt

@@ -416,6 +416,19 @@ and is between 256 and 4096 characters. It is defined in the file
 			[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

arch/x86/kernel/cpu/common.c

@@ -572,6 +572,17 @@ void __cpuinit print_cpu_info(struct cpuinfo_x86 *c)
 		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. :)

arch/x86/kernel/setup_64.c

@@ -1050,6 +1050,17 @@ void __cpuinit print_cpu_info(struct cpuinfo_x86 *c)
 		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.
  */