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>

3 files changed, 35 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 040e30a33b7c..50d564dabb13 100644
--- a/Documentation/kernel-parameters.txt
+++ b/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
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 56b7ea8e6c79..56cc341cc586 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/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. :)
diff --git a/arch/x86/kernel/setup_64.c b/arch/x86/kernel/setup_64.c
index cb9b8a90c094..79635b7bd57a 100644
--- a/arch/x86/kernel/setup_64.c
+++ b/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.
  */