From: Rahul Singh <rahul.singh@arm.com>

Add cmdline boot option "viommu = <string>" to enable or disable the
virtual iommu support for guests on ARM (only viommu="smmuv3" supported
for now).

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Signed-off-by: Milan Djokic <milan_djokic@epam.com>
---
 docs/misc/xen-command-line.pandoc      |  9 +++++++++
 xen/arch/arm/include/asm/viommu.h      | 12 ++++++++++++
 xen/drivers/passthrough/arm/viommu.c   | 11 +++++++++++
 xen/drivers/passthrough/arm/vsmmu-v3.c |  3 +++
 4 files changed, 35 insertions(+)

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 6c77129732..6531c2355c 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2850,6 +2850,15 @@ The optional `keep` parameter causes Xen to continue using the vga
 console even after dom0 has been started.  The default behaviour is to
 relinquish control to dom0.
 
+### viommu (arm)
+> `= <string>`
+
+> Default: ``
+
+Flag to enable or disable support for the virtual IOMMU for guests. Disabled by
+default. Enable by specifying target IOMMU type (if supported). Only "smmuv3"
+IOMMU emulation supported at this point.
+
 ### viridian-spinlock-retry-count (x86)
 > `= <integer>`
 
diff --git a/xen/arch/arm/include/asm/viommu.h b/xen/arch/arm/include/asm/viommu.h
index 2a6742de73..ed338fe0ec 100644
--- a/xen/arch/arm/include/asm/viommu.h
+++ b/xen/arch/arm/include/asm/viommu.h
@@ -10,6 +10,7 @@
 #include <public/xen.h>
 
 extern struct list_head host_iommu_list;
+extern char viommu[];
 
 /* data structure for each hardware IOMMU */
 struct host_iommu {
@@ -50,6 +51,12 @@ uint16_t viommu_get_type(void);
 void add_to_host_iommu_list(paddr_t addr, paddr_t size,
                             const struct dt_device_node *node);
 
+static always_inline bool is_viommu_enabled(void)
+{
+    /* only smmuv3 emulation supported */
+    return !strcmp(viommu, "smmuv3");
+}
+
 #else
 
 static inline uint8_t viommu_get_type(void)
@@ -76,6 +83,11 @@ static inline void add_to_host_iommu_list(paddr_t addr, paddr_t size,
     return;
 }
 
+static always_inline bool is_viommu_enabled(void)
+{
+    return false;
+}
+
 #endif /* CONFIG_ARM_VIRTUAL_IOMMU */
 
 #endif /* __ARCH_ARM_VIOMMU_H__ */
diff --git a/xen/drivers/passthrough/arm/viommu.c b/xen/drivers/passthrough/arm/viommu.c
index 53ae46349a..5f5892fbb2 100644
--- a/xen/drivers/passthrough/arm/viommu.c
+++ b/xen/drivers/passthrough/arm/viommu.c
@@ -3,6 +3,7 @@
 #include <xen/errno.h>
 #include <xen/init.h>
 #include <xen/irq.h>
+#include <xen/param.h>
 #include <xen/types.h>
 
 #include <asm/viommu.h>
@@ -38,8 +39,18 @@ void add_to_host_iommu_list(paddr_t addr, paddr_t size,
     list_add_tail(&iommu_data->entry, &host_iommu_list);
 }
 
+/* By default viommu is disabled.
+ * If enabled, 'viommu' param indicates type (smmuv3 is only supported type atm)
+ */
+char __read_mostly viommu[10] = "";
+string_param("viommu", viommu);
+
 int domain_viommu_init(struct domain *d, uint16_t viommu_type)
 {
+    /* Enable viommu when it has been enabled explicitly (viommu="smmuv3"). */
+    if ( !is_viommu_enabled() )
+        return 0;
+
     if ( viommu_type == XEN_DOMCTL_CONFIG_VIOMMU_NONE )
         return 0;
 
diff --git a/xen/drivers/passthrough/arm/vsmmu-v3.c b/xen/drivers/passthrough/arm/vsmmu-v3.c
index 6b4009e5ef..e36f200ba5 100644
--- a/xen/drivers/passthrough/arm/vsmmu-v3.c
+++ b/xen/drivers/passthrough/arm/vsmmu-v3.c
@@ -105,6 +105,9 @@ void __init vsmmuv3_set_type(void)
 {
     const struct viommu_desc *desc = &vsmmuv3_desc;
 
+    if ( !is_viommu_enabled() )
+        return;
+
     if ( cur_viommu && (cur_viommu != desc) )
     {
         printk("WARNING: Cannot set vIOMMU, already set to a different value\n");
-- 
2.43.0