xen/arm, doc: Add a DT property to specify IOMMU for Dom0less domUs

There are some use cases in which the dom0less domUs need to have
the XEN_DOMCTL_CDF_iommu set at the domain construction time. For
example, the dynamic dtbo feature allows the domain to be assigned
a device that is behind the IOMMU at runtime. For these use cases,
we need to have a way to specify the domain will need the IOMMU
mapping at domain construction time.

Introduce a "passthrough" DT property for Dom0less DomUs following
the same entry as the xl.cfg. Currently only provide two options,
i.e. "enable" and "disable". Set the XEN_DOMCTL_CDF_iommu at domain
construction time based on the property.

Signed-off-by: Henry Wang <xin.wang2@amd.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>

docs/misc/arm/device-tree/booting.txt

@@ -260,6 +260,22 @@ with the following properties:
     value specified by Xen command line parameter gnttab_max_maptrack_frames
     (or its default value if unspecified, i.e. 1024) is used.
 
+- passthrough
+
+    A string property specifying whether IOMMU mappings are enabled for the
+    domain and hence whether it will be enabled for passthrough hardware.
+    Possible property values are:
+
+    - "enabled"
+    IOMMU mappings are enabled for the domain. Note that this option is the
+    default if the user provides the device partial passthrough device tree
+    for the domain.
+
+    - "disabled"
+    IOMMU mappings are disabled for the domain and so hardware may not be
+    passed through. This option is the default if this property is missing
+    and the user does not provide the device partial device tree for the domain.
+
 Under the "xen,domain" compatible node, one or more sub-nodes are present
 for the DomU kernel and ramdisk.
 

xen/arch/arm/dom0less-build.c

@@ -848,6 +848,8 @@ static int __init construct_domU(struct domain *d,
 void __init create_domUs(void)
 {
     struct dt_device_node *node;
+    const char *dom0less_iommu;
+    bool iommu = false;
     const struct dt_device_node *cpupool_node,
                                 *chosen = dt_find_node_by_path("/chosen");
 
@@ -895,8 +897,13 @@ void __init create_domUs(void)
             panic("Missing property 'cpus' for domain %s\n",
                   dt_node_name(node));
 
-        if ( dt_find_compatible_node(node, NULL, "multiboot,device-tree") &&
-             iommu_enabled )
+        if ( !dt_property_read_string(node, "passthrough", &dom0less_iommu) &&
+             !strcmp(dom0less_iommu, "enabled") )
+            iommu = true;
+
+        if ( iommu_enabled &&
+             (iommu || dt_find_compatible_node(node, NULL,
+                                               "multiboot,device-tree")) )
             d_cfg.flags |= XEN_DOMCTL_CDF_iommu;
 
         if ( !dt_property_read_u32(node, "nr_spis", &d_cfg.arch.nr_spis) )