x86: disable intel_iommu support by default

Due to recurring issues with DMAR support on certain platforms.
There's a number of filesystem corruption incidents reported:

  https://bugzilla.redhat.com/show_bug.cgi?id=479996
  http://bugzilla.kernel.org/show_bug.cgi?id=12578

Provide a Kconfig option to change whether it is enabled by
default.

If disabled, it can still be reenabled by passing intel_iommu=on to the
kernel. Keep the .config option off by default.
Signed-off-by: Kyle McMartin <kyle@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-By: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Documentation/kernel-parameters.txt

@@ -937,6 +937,8 @@ and is between 256 and 4096 characters. It is defined in the file
 
 
 	intel_iommu=	[DMAR] Intel IOMMU driver (DMAR) option
+		on
+			Enable intel iommu driver.
 		off
 			Disable intel iommu driver.
 		igfx_off [Default Off]

arch/x86/Kconfig

@@ -1802,6 +1802,17 @@ config DMAR
 	  and include PCI device scope covered by these DMA
 	  remapping devices.
 
+config DMAR_DEFAULT_ON
+	def_bool n
+	prompt "Enable DMA Remapping Devices by default"
+	depends on DMAR
+	help
+	  Selecting this option will enable a DMAR device at boot time if
+	  one is found. If this option is not selected, DMAR support can
+	  be enabled by passing intel_iommu=on to the kernel. It is
+	  recommended you say N here while the DMAR code remains
+	  experimental.
+
 config DMAR_GFX_WA
 	def_bool y
 	prompt "Support for Graphics workaround"

drivers/pci/intel-iommu.c

@@ -268,7 +268,12 @@ static long list_size;
 
 static void domain_remove_dev_info(struct dmar_domain *domain);
 
-int dmar_disabled;
+#ifdef CONFIG_DMAR_DEFAULT_ON
+int dmar_disabled = 0;
+#else
+int dmar_disabled = 1;
+#endif /*CONFIG_DMAR_DEFAULT_ON*/
+
 static int __initdata dmar_map_gfx = 1;
 static int dmar_forcedac;
 static int intel_iommu_strict;
@@ -284,9 +289,12 @@ static int __init intel_iommu_setup(char *str)
 	if (!str)
 		return -EINVAL;
 	while (*str) {
-		if (!strncmp(str, "off", 3)) {
+		if (!strncmp(str, "on", 2)) {
+			dmar_disabled = 0;
+			printk(KERN_INFO "Intel-IOMMU: enabled\n");
+		} else if (!strncmp(str, "off", 3)) {
 			dmar_disabled = 1;
-			printk(KERN_INFO"Intel-IOMMU: disabled\n");
+			printk(KERN_INFO "Intel-IOMMU: disabled\n");
 		} else if (!strncmp(str, "igfx_off", 8)) {
 			dmar_map_gfx = 0;
 			printk(KERN_INFO