[PATCH v2 1/2] xen: add knownalive_domain_from_domid() helper

[Juergen Gross <jgross@suse.com>]

Add a helper knownalive_domain_from_domid() returning the struct domain
pointer for a domain give by its domid and which is known not being
able to be released (its reference count isn't incremented and no
rcu_lock_domain() is called for it).

In order to simplify coding add an internal helper for doing the lookup
and call that from the new function and similar functions.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- rename helper to knownalive_domain_from_domid() (Jan Beulich)
- enhance comment in header (Jan Beulich)
- rename internal helper (Julien Grall)
---
 xen/common/domain.c     | 53 +++++++++++++++++++++++++----------------
 xen/include/xen/sched.h |  4 ++++
 2 files changed, 37 insertions(+), 20 deletions(-)

diff --git a/xen/common/domain.c b/xen/common/domain.c
index 8dd6cd5a8f..35e0dc5139 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -835,25 +835,32 @@ out:
     return 0;
 }
 
-
-struct domain *get_domain_by_id(domid_t dom)
+/* rcu_read_lock(&domlist_read_lock) must be held. */
+static struct domain *domid_to_domain(domid_t dom)
 {
     struct domain *d;
 
-    rcu_read_lock(&domlist_read_lock);
-
     for ( d = rcu_dereference(domain_hash[DOMAIN_HASH(dom)]);
           d != NULL;
           d = rcu_dereference(d->next_in_hashbucket) )
     {
         if ( d->domain_id == dom )
-        {
-            if ( unlikely(!get_domain(d)) )
-                d = NULL;
-            break;
-        }
+            return d;
     }
 
+    return NULL;
+}
+
+struct domain *get_domain_by_id(domid_t dom)
+{
+    struct domain *d;
+
+    rcu_read_lock(&domlist_read_lock);
+
+    d = domid_to_domain(dom);
+    if ( d && unlikely(!get_domain(d)) )
+        d = NULL;
+
     rcu_read_unlock(&domlist_read_lock);
 
     return d;
@@ -862,20 +869,26 @@ struct domain *get_domain_by_id(domid_t dom)
 
 struct domain *rcu_lock_domain_by_id(domid_t dom)
 {
-    struct domain *d = NULL;
+    struct domain *d;
 
     rcu_read_lock(&domlist_read_lock);
 
-    for ( d = rcu_dereference(domain_hash[DOMAIN_HASH(dom)]);
-          d != NULL;
-          d = rcu_dereference(d->next_in_hashbucket) )
-    {
-        if ( d->domain_id == dom )
-        {
-            rcu_lock_domain(d);
-            break;
-        }
-    }
+    d = domid_to_domain(dom);
+    if ( d )
+        rcu_lock_domain(d);
+
+    rcu_read_unlock(&domlist_read_lock);
+
+    return d;
+}
+
+struct domain *knownalive_domain_from_domid(domid_t dom)
+{
+    struct domain *d;
+
+    rcu_read_lock(&domlist_read_lock);
+
+    d = domid_to_domain(dom);
 
     rcu_read_unlock(&domlist_read_lock);
 
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 557b3229f6..9e9c3d834b 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -737,8 +737,12 @@ static inline struct domain *rcu_lock_current_domain(void)
     return /*rcu_lock_domain*/(current->domain);
 }
 
+/* Get struct domain AND increase ref-count of domain. */
 struct domain *get_domain_by_id(domid_t dom);
 
+/* Get struct domain known to have reference held or being RCU-locked. */
+struct domain *knownalive_domain_from_domid(domid_t dom);
+
 struct domain *get_pg_owner(domid_t domid);
 
 static inline void put_pg_owner(struct domain *pg_owner)
-- 
2.35.3