x86: update GADDR based secondary time area

Before adding a new vCPU operation to register the secondary time area
by guest-physical address, add code to actually keep such areas up-to-
date.

Note that pages aren't marked dirty when written to (matching the
handling of space mapped by map_vcpu_info()), on the basis that the
registrations are lost anyway across migration (or would need re-
populating at the target for transparent migration). Plus the contents
of the areas in question have to be deemed volatile in the first place
(so saving a "most recent" value is pretty meaningless even for e.g.
snapshotting).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

xen/arch/x86/time.c

@@ -1566,12 +1566,34 @@ static void __update_vcpu_system_time(struct vcpu *v, int force)
         v->arch.pv.pending_system_time = _u;
 }
 
+static void write_time_guest_area(struct vcpu_time_info *map,
+                                  const struct vcpu_time_info *src)
+{
+    /* 1. Update userspace version. */
+    write_atomic(&map->version, src->version);
+    smp_wmb();
+
+    /* 2. Update all other userspace fields. */
+    *map = *src;
+
+    /* 3. Update userspace version again. */
+    smp_wmb();
+    write_atomic(&map->version, version_update_end(src->version));
+}
+
 bool update_secondary_system_time(struct vcpu *v,
                                   struct vcpu_time_info *u)
 {
     XEN_GUEST_HANDLE(vcpu_time_info_t) user_u = v->arch.time_info_guest;
+    struct vcpu_time_info *map = v->arch.time_guest_area.map;
     struct guest_memory_policy policy = { .nested_guest_mode = false };
 
+    if ( map )
+    {
+        write_time_guest_area(map, u);
+        return true;
+    }
+
     if ( guest_handle_is_null(user_u) )
         return true;