iommu/vt-d: Enable Intel IOMMU scalable mode by default

The commit 8950dcd8 ("iommu/vt-d: Leave scalable mode default off")
leaves the scalable mode default off and end users could turn it on with
"intel_iommu=sm_on". Using the Intel IOMMU scalable mode for kernel DMA,
user-level device access and Shared Virtual Address have been enabled.
This enables the scalable mode by default if the hardware advertises the
support and adds kernel options of "intel_iommu=sm_on/sm_off" for end
users to configure it through the kernel parameters.
Suggested-by: Ashok Raj <ashok.raj@intel.com>
Suggested-by: Sanjay Kumar <sanjay.k.kumar@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20210720013856.4143880-1-baolu.lu@linux.intel.com
Link: https://lore.kernel.org/r/20210818134852.1847070-5-baolu.lu@linux.intel.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

Documentation/admin-guide/kernel-parameters.txt

@@ -1946,11 +1946,12 @@
 			By default, super page will be supported if Intel IOMMU
 			has the capability. With this option, super page will
 			not be supported.
-		sm_on [Default Off]
-			By default, scalable mode will be disabled even if the
-			hardware advertises that it has support for the scalable
-			mode translation. With this option set, scalable mode
-			will be used on hardware which claims to support it.
+		sm_on
+			Enable the Intel IOMMU scalable mode if the hardware
+			advertises that it has support for the scalable mode
+			translation.
+		sm_off
+			Disallow use of the Intel IOMMU scalable mode.
 		tboot_noforce [Default Off]
 			Do not force the Intel IOMMU enabled under tboot.
 			By default, tboot will force Intel IOMMU on, which

drivers/iommu/intel/Kconfig

@@ -84,6 +84,7 @@ config INTEL_IOMMU_FLOPPY_WA
 
 config INTEL_IOMMU_SCALABLE_MODE_DEFAULT_ON
 	bool "Enable Intel IOMMU scalable mode by default"
+	default y
 	help
 	  Selecting this option will enable by default the scalable mode if
 	  hardware presents the capability. The scalable mode is defined in

drivers/iommu/intel/iommu.c

@@ -432,8 +432,11 @@ static int __init intel_iommu_setup(char *str)
 			pr_info("Disable supported super page\n");
 			intel_iommu_superpage = 0;
 		} else if (!strncmp(str, "sm_on", 5)) {
-			pr_info("Intel-IOMMU: scalable mode supported\n");
+			pr_info("Enable scalable mode if hardware supports\n");
 			intel_iommu_sm = 1;
+		} else if (!strncmp(str, "sm_off", 6)) {
+			pr_info("Scalable mode is disallowed\n");
+			intel_iommu_sm = 0;
 		} else if (!strncmp(str, "tboot_noforce", 13)) {
 			pr_info("Intel-IOMMU: not forcing on after tboot. This could expose security risk for tboot\n");
 			intel_iommu_tboot_noforce = 1;