Add cmdline boot option "viommu = <boolean>" to enable or disable the
virtual iommu support for guests on ARM.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
---
 docs/misc/xen-command-line.pandoc      |  7 +++++++
 xen/arch/arm/include/asm/viommu.h      | 11 +++++++++++
 xen/drivers/passthrough/arm/viommu.c   |  9 +++++++++
 xen/drivers/passthrough/arm/vsmmu-v3.c |  3 +++
 4 files changed, 30 insertions(+)

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 424b12cfb2..14a104f2b9 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -1896,6 +1896,13 @@ This option can be specified more than once (up to 8 times at present).
 
 Flag to enable or disable support for PCI passthrough
 
+### viommu (arm)
+> `= <boolean>`
+
+> Default: `false`
+
+Flag to enable or disable support for Virtual IOMMU for guests.
+
 ### pcid (x86)
 > `= <boolean> | xpti=<bool>`
 
diff --git a/xen/arch/arm/include/asm/viommu.h b/xen/arch/arm/include/asm/viommu.h
index 4785877e2a..4de4cceeda 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 bool viommu_enabled;
 
 /* data structure for each hardware IOMMU */
 struct host_iommu {
@@ -50,6 +51,11 @@ 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)
+{
+    return viommu_enabled;
+}
+
 #else
 
 static inline uint8_t viommu_get_type(void)
@@ -76,6 +82,11 @@ 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_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..a1d6a04ba9 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 +39,16 @@ 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. */
+bool __read_mostly viommu_enabled;
+boolean_param("viommu", viommu_enabled);
+
 int domain_viommu_init(struct domain *d, uint16_t viommu_type)
 {
+    /* Enable viommu when it has been enabled explicitly (viommu=on). */
+    if ( !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");
-- 
2.25.1

On 01.12.2022 17:02, Rahul Singh wrote:
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -1896,6 +1896,13 @@ This option can be specified more than once (up to 8 times at present).
>  
>  Flag to enable or disable support for PCI passthrough
>  
> +### viommu (arm)
> +> `= <boolean>`
> +
> +> Default: `false`
> +
> +Flag to enable or disable support for Virtual IOMMU for guests.
> +
>  ### pcid (x86)
>  > `= <boolean> | xpti=<bool>`

I'm sorry for being blunt, but how did you select this particular slot
for insertion, when the whole file clearly at least tries to have options
sorted alphabetically.

Apart from that I wonder whether this actually needs a command line
control, when supposedly this is controlled on a per-domain basis anyway.

Jan