[PATCH 1/2] x86: Perform mem_sharing teardown before paging teardown

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

An assert failure has been observed at p2m-basic.c:173 when performing vm
forking and then destroying the forked VM. The assert checks whether the
domain's shared pages counter is 0. According to the patch that originally
added the assert (7bedbbb5c31) the p2m_teardown should only happen after
mem_sharing already relinquished all shared pages.

In this patch we flip the order in which relinquish ops are called to avoid
tripping the assert.

Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
---
Note: it is unclear why this assert hasn't tripped in the past. It hasn't
been observed with 4.17-rc1 but it is in RELEASE-4.17.0
---
 xen/arch/x86/domain.c | 52 +++++++++++++++++++++----------------------
 1 file changed, 26 insertions(+), 26 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index db3ebf062d..453ec52b6a 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2310,6 +2310,32 @@ int domain_relinquish_resources(struct domain *d)
         if ( ret )
             return ret;
 
+#ifdef CONFIG_MEM_SHARING
+    PROGRESS(shared):
+
+        if ( is_hvm_domain(d) )
+        {
+            /* If the domain has shared pages, relinquish them allowing
+             * for preemption. */
+            ret = relinquish_shared_pages(d);
+            if ( ret )
+                return ret;
+
+            /*
+             * If the domain is forked, decrement the parent's pause count
+             * and release the domain.
+             */
+            if ( mem_sharing_is_fork(d) )
+            {
+                struct domain *parent = d->parent;
+
+                d->parent = NULL;
+                domain_unpause(parent);
+                put_domain(parent);
+            }
+        }
+#endif
+
     PROGRESS(paging):
 
         /* Tear down paging-assistance stuff. */
@@ -2350,32 +2376,6 @@ int domain_relinquish_resources(struct domain *d)
             d->arch.auto_unmask = 0;
         }
 
-#ifdef CONFIG_MEM_SHARING
-    PROGRESS(shared):
-
-        if ( is_hvm_domain(d) )
-        {
-            /* If the domain has shared pages, relinquish them allowing
-             * for preemption. */
-            ret = relinquish_shared_pages(d);
-            if ( ret )
-                return ret;
-
-            /*
-             * If the domain is forked, decrement the parent's pause count
-             * and release the domain.
-             */
-            if ( mem_sharing_is_fork(d) )
-            {
-                struct domain *parent = d->parent;
-
-                d->parent = NULL;
-                domain_unpause(parent);
-                put_domain(parent);
-            }
-        }
-#endif
-
         spin_lock(&d->page_alloc_lock);
         page_list_splice(&d->arch.relmem_list, &d->page_list);
         INIT_PAGE_LIST_HEAD(&d->arch.relmem_list);
-- 
2.34.1