x86/time: introduce helper to fetch Xen wallclock when running as a guest

Move the current code in get_wallclock_time() to fetch the Xen wallclock
information from the shared page when running as a guest into a separate
helper.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/time.c

@@ -787,6 +787,30 @@ static struct platform_timesource __initdata_cf_clobber plt_xen_timer =
 };
 #endif
 
+static unsigned long read_xen_wallclock(void)
+{
+#ifdef CONFIG_XEN_GUEST
+    const struct shared_info *sh_info = XEN_shared_info;
+    uint32_t wc_version;
+    uint64_t wc_sec;
+
+    ASSERT(xen_guest);
+
+    do {
+        wc_version = sh_info->wc_version & ~1;
+        smp_rmb();
+
+        wc_sec  = sh_info->wc_sec;
+        smp_rmb();
+    } while ( wc_version != sh_info->wc_version );
+
+    return wc_sec + read_xen_timer() / 1000000000;
+#else
+    ASSERT_UNREACHABLE();
+    return 0;
+#endif
+}
+
 #ifdef CONFIG_HYPERV_GUEST
 /************************************************************
  * HYPER-V REFERENCE TSC
@@ -1497,24 +1521,8 @@ void rtc_guest_write(unsigned int port, unsigned int data)
 
 static unsigned long get_wallclock_time(void)
 {
-#ifdef CONFIG_XEN_GUEST
     if ( xen_guest )
-    {
-        struct shared_info *sh_info = XEN_shared_info;
-        uint32_t wc_version;
-        uint64_t wc_sec;
-
-        do {
-            wc_version = sh_info->wc_version & ~1;
-            smp_rmb();
-
-            wc_sec  = sh_info->wc_sec;
-            smp_rmb();
-        } while ( wc_version != sh_info->wc_version );
-
-        return wc_sec + read_xen_timer() / 1000000000;
-    }
-#endif
+        return read_xen_wallclock();
 
     return get_cmos_time();
 }