arm/p2m: Rework p2m_init()

p2m_init() is mostly trivial initialisation, but has two fallible operations
which are on either side of the backpointer trigger for teardown to take
actions.

p2m_free_vmid() is idempotent with a failed p2m_alloc_vmid(), so rearrange
p2m_init() to perform all trivial setup, then set the backpointer, then
perform all fallible setup.

This will simplify a future bugfix which needs to add a third fallible
operation.

No practical change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

xen/arch/arm/p2m.c

@@ -1754,7 +1754,7 @@ void p2m_final_teardown(struct domain *d)
 int p2m_init(struct domain *d)
 {
     struct p2m_domain *p2m = p2m_get_hostp2m(d);
-    int rc = 0;
+    int rc;
     unsigned int cpu;
 
     rwlock_init(&p2m->lock);
@@ -1763,11 +1763,6 @@ int p2m_init(struct domain *d)
     INIT_PAGE_LIST_HEAD(&d->arch.paging.p2m_freelist);
 
     p2m->vmid = INVALID_VMID;
-
-    rc = p2m_alloc_vmid(d);
-    if ( rc != 0 )
-        return rc;
-
     p2m->max_mapped_gfn = _gfn(0);
     p2m->lowest_mapped_gfn = _gfn(ULONG_MAX);
 
@@ -1783,8 +1778,6 @@ int p2m_init(struct domain *d)
     p2m->clean_pte = is_iommu_enabled(d) &&
         !iommu_has_feature(d, IOMMU_FEAT_COHERENT_WALK);
 
-    rc = p2m_alloc_table(d);
-
     /*
      * Make sure that the type chosen to is able to store the an vCPU ID
      * between 0 and the maximum of virtual CPUS supported as long as
@@ -1797,13 +1790,20 @@ int p2m_init(struct domain *d)
        p2m->last_vcpu_ran[cpu] = INVALID_VCPU_ID;
 
     /*
-     * Besides getting a domain when we only have the p2m in hand,
-     * the back pointer to domain is also used in p2m_teardown()
-     * as an end-of-initialization indicator.
+     * "Trivial" initialisation is now complete.  Set the backpointer so
+     * p2m_teardown() and friends know to do something.
      */
     p2m->domain = d;
 
-    return rc;
+    rc = p2m_alloc_vmid(d);
+    if ( rc )
+        return rc;
+
+    rc = p2m_alloc_table(d);
+    if ( rc )
+        return rc;
+
+    return 0;
 }
 
 /*