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>

3 files changed, 24 insertions, 3 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index d8362cf9909e..b182626739ea 100644
--- a/Documentation/kernel-parameters.txt
+++ b/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]
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 73f7fe8fd4d1..9c39095b33fc 100644
--- a/arch/x86/Kconfig
+++ b/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"
diff --git a/drivers/pci/intel-iommu.c b/drivers/pci/intel-iommu.c
index 3dfecb20d5e7..f4b7c79023ff 100644
--- a/drivers/pci/intel-iommu.c
+++ b/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