common/device_tree: Add rwlock for dt_host

Dynamic programming ops will modify the dt_host and there might be other
functions which are browsing the dt_host at the same time. To avoid the race
conditions, adding rwlock for browsing the dt_host during runtime. dt_host
writer will be added in the follow-up patch for device tree overlay
functionalities.

Reason behind adding rwlock instead of spinlock:
    For now, dynamic programming is the sole modifier of dt_host in Xen during
    run time. All other access functions like iommu_release_dt_device() are
    just reading the dt_host during run-time. So, there is a need to protect
    others from browsing the dt_host while dynamic programming is modifying
    it. rwlock is better suitable for this task as spinlock won't be able to
    differentiate between read and write access.

Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

xen/common/device_tree.c

@@ -31,6 +31,7 @@ dt_irq_xlate_func dt_irq_xlate;
 struct dt_device_node *dt_host;
 /* Interrupt controller node*/
 const struct dt_device_node *dt_interrupt_controller;
+DEFINE_RWLOCK(dt_host_lock);
 
 /**
  * struct dt_alias_prop - Alias property in 'aliases' node

xen/drivers/passthrough/device_tree.c

@@ -31,6 +31,8 @@ int iommu_assign_dt_device(struct domain *d, struct dt_device_node *dev)
     int rc = -EBUSY;
     struct domain_iommu *hd = dom_iommu(d);
 
+    ASSERT(system_state < SYS_STATE_active || rw_is_locked(&dt_host_lock));
+
     if ( !is_iommu_enabled(d) )
         return -EINVAL;
 
@@ -62,6 +64,8 @@ int iommu_deassign_dt_device(struct domain *d, struct dt_device_node *dev)
     const struct domain_iommu *hd = dom_iommu(d);
     int rc;
 
+    ASSERT(rw_is_locked(&dt_host_lock));
+
     if ( !is_iommu_enabled(d) )
         return -EINVAL;
 
@@ -113,6 +117,8 @@ int iommu_release_dt_devices(struct domain *d)
     if ( !is_iommu_enabled(d) )
         return 0;
 
+    read_lock(&dt_host_lock);
+
     list_for_each_entry_safe(dev, _dev, &hd->dt_devices, domain_list)
     {
         rc = iommu_deassign_dt_device(d, dev);
@@ -120,10 +126,14 @@ int iommu_release_dt_devices(struct domain *d)
         {
             dprintk(XENLOG_ERR, "Failed to deassign %s in domain %u\n",
                     dt_node_full_name(dev), d->domain_id);
+            read_unlock(&dt_host_lock);
+
             return rc;
         }
     }
 
+    read_unlock(&dt_host_lock);
+
     return 0;
 }
 
@@ -133,6 +143,8 @@ int iommu_remove_dt_device(struct dt_device_node *np)
     struct device *dev = dt_to_dev(np);
     int rc;
 
+    ASSERT(rw_is_locked(&dt_host_lock));
+
     if ( !iommu_enabled )
         return 1;
 
@@ -177,6 +189,8 @@ int iommu_add_dt_device(struct dt_device_node *np)
     struct device *dev = dt_to_dev(np);
     int rc = 1, index = 0;
 
+    ASSERT(system_state < SYS_STATE_active || rw_is_locked(&dt_host_lock));
+
     if ( !iommu_enabled )
         return 1;
 
@@ -249,6 +263,8 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct domain *d,
     int ret;
     struct dt_device_node *dev;
 
+    read_lock(&dt_host_lock);
+
     switch ( domctl->cmd )
     {
     case XEN_DOMCTL_assign_device:
@@ -289,7 +305,10 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct domain *d,
         }
 
         if ( d == dom_io )
-            return -EINVAL;
+        {
+            ret = -EINVAL;
+            break;
+        }
 
         ret = iommu_add_dt_device(dev);
         if ( ret < 0 )
@@ -327,7 +346,10 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct domain *d,
             break;
 
         if ( d == dom_io )
-            return -EINVAL;
+        {
+            ret = -EINVAL;
+            break;
+        }
 
         ret = iommu_deassign_dt_device(d, dev);
 
@@ -342,5 +364,7 @@ int iommu_do_dt_domctl(struct xen_domctl *domctl, struct domain *d,
         break;
     }
 
+    read_unlock(&dt_host_lock);
+
     return ret;
 }

xen/include/xen/device_tree.h

@@ -18,6 +18,7 @@
 #include <xen/string.h>
 #include <xen/types.h>
 #include <xen/list.h>
+#include <xen/rwlock.h>
 
 #define DEVICE_TREE_MAX_DEPTH 16
 
@@ -218,6 +219,12 @@ extern struct dt_device_node *dt_host;
  */
 extern const struct dt_device_node *dt_interrupt_controller;
 
+/*
+ * Lock that protects r/w updates to unflattened device tree i.e. dt_host during
+ * runtime. Lock may not be taken for boot only code.
+ */
+extern rwlock_t dt_host_lock;
+
 /**
  * Find the interrupt controller
  * For the moment we handle only one interrupt controller: the first