iommu: Add config option to set passthrough as default

This allows the default behavior to be controlled by a kernel config
option instead of changing the commandline for the kernel to include
"iommu.passthrough=on" or "iommu=pt" on machines where this is desired.

Likewise, for machines where this config option is enabled, it can be
disabled at boot time with "iommu.passthrough=off" or "iommu=nopt".

Also corrected iommu=pt documentation for IA-64, since it has no code that
parses iommu= at all.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Documentation/admin-guide/kernel-parameters.txt

@@ -1716,7 +1716,8 @@
 		merge
 		nomerge
 		soft
-		pt		[x86, IA-64]
+		pt		[x86]
+		nopt		[x86]
 		nobypass	[PPC/POWERNV]
 			Disable IOMMU bypass, using IOMMU for PCI devices.
 

arch/x86/kernel/pci-dma.c

@@ -40,8 +40,14 @@ int iommu_detected __read_mostly = 0;
  * devices and allow every device to access to whole physical memory. This is
  * useful if a user wants to use an IOMMU only for KVM device assignment to
  * guests and not for driver dma translation.
+ * It is also possible to disable by default in kernel config, and enable with
+ * iommu=nopt at boot time.
  */
+#ifdef CONFIG_IOMMU_DEFAULT_PASSTHROUGH
+int iommu_pass_through __read_mostly = 1;
+#else
 int iommu_pass_through __read_mostly;
+#endif
 
 extern struct iommu_table_entry __iommu_table[], __iommu_table_end[];
 
@@ -135,6 +141,8 @@ static __init int iommu_setup(char *p)
 #endif
 		if (!strncmp(p, "pt", 2))
 			iommu_pass_through = 1;
+		if (!strncmp(p, "nopt", 4))
+			iommu_pass_through = 0;
 
 		gart_parse_options(p);
 

drivers/iommu/Kconfig

@@ -70,6 +70,17 @@ config IOMMU_DEBUGFS
 	  debug/iommu directory, and then populate a subdirectory with
 	  entries as required.
 
+config IOMMU_DEFAULT_PASSTHROUGH
+	bool "IOMMU passthrough by default"
+	depends on IOMMU_API
+        help
+	  Enable passthrough by default, removing the need to pass in
+	  iommu.passthrough=on or iommu=pt through command line. If this
+	  is enabled, you can still disable with iommu.passthrough=off
+	  or iommu=nopt depending on the architecture.
+
+	  If unsure, say N here.
+
 config IOMMU_IOVA
 	tristate
 

drivers/iommu/iommu.c

@@ -36,7 +36,11 @@
 
 static struct kset *iommu_group_kset;
 static DEFINE_IDA(iommu_group_ida);
+#ifdef CONFIG_IOMMU_DEFAULT_PASSTHROUGH
+static unsigned int iommu_def_domain_type = IOMMU_DOMAIN_IDENTITY;
+#else
 static unsigned int iommu_def_domain_type = IOMMU_DOMAIN_DMA;
+#endif
 
 struct iommu_callback_data {
 	const struct iommu_ops *ops;