x86/PV: fix unintended dependency of m2p-strict mode on migration-v2

This went unnoticed until a backport of this to an older Xen got used,
causing migration of guests enabling this VM assist to fail, because
page table pinning there preceeds vCPU context loading, and hence L4
tables get initialized for the wrong mode. Fix this by post-processing
L4 tables when setting the intended VM assist flags for the guest.

Note that this leaves in place a dependency on vCPU 0 getting its guest
context restored first, but afaict the logic here is not the only thing
depending on that.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1067,8 +1067,48 @@ int arch_set_info_guest(
         goto out;
 
     if ( v->vcpu_id == 0 )
+    {
         d->vm_assist = c(vm_assist);
 
+        /*
+         * In the restore case we need to deal with L4 pages which got
+         * initialized with m2p_strict still clear (and which hence lack the
+         * correct initial RO_MPT_VIRT_{START,END} L4 entry).
+         */
+        if ( d != current->domain && VM_ASSIST(d, m2p_strict) &&
+             is_pv_domain(d) && !is_pv_32bit_domain(d) &&
+             atomic_read(&d->arch.pv_domain.nr_l4_pages) )
+        {
+            bool_t done = 0;
+
+            spin_lock_recursive(&d->page_alloc_lock);
+
+            for ( i = 0; ; )
+            {
+                struct page_info *page = page_list_remove_head(&d->page_list);
+
+                if ( page_lock(page) )
+                {
+                    if ( (page->u.inuse.type_info & PGT_type_mask) ==
+                         PGT_l4_page_table )
+                        done = !fill_ro_mpt(page_to_mfn(page));
+
+                    page_unlock(page);
+                }
+
+                page_list_add_tail(page, &d->page_list);
+
+                if ( done || (!(++i & 0xff) && hypercall_preempt_check()) )
+                    break;
+            }
+
+            spin_unlock_recursive(&d->page_alloc_lock);
+
+            if ( !done )
+                return -ERESTART;
+        }
+    }
+
     rc = put_old_guest_table(current);
     if ( rc )
         return rc;
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1463,13 +1463,20 @@ void init_guest_l4_table(l4_pgentry_t l4
         l4tab[l4_table_offset(RO_MPT_VIRT_START)] = l4e_empty();
 }
 
-void fill_ro_mpt(unsigned long mfn)
+bool_t fill_ro_mpt(unsigned long mfn)
 {
     l4_pgentry_t *l4tab = map_domain_page(_mfn(mfn));
+    bool_t ret = 0;
 
-    l4tab[l4_table_offset(RO_MPT_VIRT_START)] =
-        idle_pg_table[l4_table_offset(RO_MPT_VIRT_START)];
+    if ( !l4e_get_intpte(l4tab[l4_table_offset(RO_MPT_VIRT_START)]) )
+    {
+        l4tab[l4_table_offset(RO_MPT_VIRT_START)] =
+            idle_pg_table[l4_table_offset(RO_MPT_VIRT_START)];
+        ret = 1;
+    }
     unmap_domain_page(l4tab);
+
+    return ret;
 }
 
 void zap_ro_mpt(unsigned long mfn)
@@ -1527,10 +1534,15 @@ static int alloc_l4_table(struct page_in
         adjust_guest_l4e(pl4e[i], d);
     }
 
-    init_guest_l4_table(pl4e, d, !VM_ASSIST(d, m2p_strict));
+    if ( rc >= 0 )
+    {
+        init_guest_l4_table(pl4e, d, !VM_ASSIST(d, m2p_strict));
+        atomic_inc(&d->arch.pv_domain.nr_l4_pages);
+        rc = 0;
+    }
     unmap_domain_page(pl4e);
 
-    return rc > 0 ? 0 : rc;
+    return rc;
 }
 
 static void free_l1_table(struct page_info *page)
@@ -1648,7 +1660,13 @@ static int free_l4_table(struct page_inf
 
     unmap_domain_page(pl4e);
 
-    return rc > 0 ? 0 : rc;
+    if ( rc >= 0 )
+    {
+        atomic_dec(&d->arch.pv_domain.nr_l4_pages);
+        rc = 0;
+    }
+
+    return rc;
 }
 
 int page_lock(struct page_info *page)
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -248,6 +248,8 @@ struct pv_domain
 {
     l1_pgentry_t **gdt_ldt_l1tab;
 
+    atomic_t nr_l4_pages;
+
     /* map_domain_page() mapping cache. */
     struct mapcache_domain mapcache;
 };
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -322,7 +322,7 @@ int free_page_type(struct page_info *pag
 
 void init_guest_l4_table(l4_pgentry_t[], const struct domain *,
                          bool_t zap_ro_mpt);
-void fill_ro_mpt(unsigned long mfn);
+bool_t fill_ro_mpt(unsigned long mfn);
 void zap_ro_mpt(unsigned long mfn);
 
 int is_iomem_page(unsigned long mfn);