qga: Don't require 'time' argument in guest-set-time command

As the description to the guest-set-time states, the command is
there to ease time synchronization after resume. If guest was
suspended for longer period of time, its system time can go off
so badly, that even NTP refuses to set it. That's why the command
was invented: to give users chance to set the time (not
necessarily 100% correct). However, there's is no real need for
us to require users to pass an arbitrary time. Especially if we
can read the correct value from RTC (boiling down to reading
host's time). Hence this commit enables logic:

guest-set-time() == guest-set-time($now_from_rtc)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qga/commands-posix.c

@@ -142,7 +142,7 @@ int64_t qmp_guest_get_time(Error **errp)
    return time_ns;
 }
 
-void qmp_guest_set_time(int64_t time_ns, Error **errp)
+void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
 {
     int ret;
     int status;
@@ -150,6 +150,8 @@ void qmp_guest_set_time(int64_t time_ns, Error **errp)
     Error *local_err = NULL;
     struct timeval tv;
 
+    /* If user has passed a time, validate and set it. */
+    if (has_time) {
         /* year-2038 will overflow in case time_t is 32bit */
         if (time_ns / 1000000000 != (time_t)(time_ns / 1000000000)) {
             error_setg(errp, "Time %" PRId64 " is too large", time_ns);
@@ -164,8 +166,12 @@ void qmp_guest_set_time(int64_t time_ns, Error **errp)
             error_setg_errno(errp, errno, "Failed to set time to guest");
             return;
         }
+    }
 
-    /* Set the Hardware Clock to the current System Time. */
+    /* Now, if user has passed a time to set and the system time is set, we
+     * just need to synchronize the hardware clock. However, if no time was
+     * passed, user is requesting the opposite: set the system time from the
+     * hardware clock. */
     pid = fork();
     if (pid == 0) {
         setsid();
@@ -173,7 +179,10 @@ void qmp_guest_set_time(int64_t time_ns, Error **errp)
         reopen_fd_to_null(1);
         reopen_fd_to_null(2);
 
-        execle("/sbin/hwclock", "hwclock", "-w", NULL, environ);
+        /* Use '/sbin/hwclock -w' to set RTC from the system time,
+         * or '/sbin/hwclock -s' to set the system time from RTC. */
+        execle("/sbin/hwclock", "hwclock", has_time ? "-w" : "-s",
+               NULL, environ);
         _exit(EXIT_FAILURE);
     } else if (pid < 0) {
         error_setg_errno(errp, errno, "failed to create child process");

qga/commands-win32.c

@@ -370,12 +370,14 @@ int64_t qmp_guest_get_time(Error **errp)
     return time_ns;
 }
 
-void qmp_guest_set_time(int64_t time_ns, Error **errp)
+void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
 {
     SYSTEMTIME ts;
     FILETIME tf;
     LONGLONG time;
 
+    if (has_time) {
+        /* Okay, user passed a time to set. Validate it. */
         if (time_ns < 0 || time_ns / 100 > INT64_MAX - W32_FT_OFFSET) {
             error_setg(errp, "Time %" PRId64 "is invalid", time_ns);
             return;
@@ -387,9 +389,19 @@ void qmp_guest_set_time(int64_t time_ns, Error **errp)
         tf.dwHighDateTime = (DWORD) (time >> 32);
 
         if (!FileTimeToSystemTime(&tf, &ts)) {
-        error_setg(errp, "Failed to convert system time %d", (int)GetLastError());
+            error_setg(errp, "Failed to convert system time %d",
+                       (int)GetLastError());
             return;
         }
+    } else {
+        /* Otherwise read the time from RTC which contains the correct value.
+         * Hopefully. */
+        GetSystemTime(&ts);
+        if (ts.wYear < 1601 || ts.wYear > 30827) {
+            error_setg(errp, "Failed to get time");
+            return;
+        }
+    }
 
     acquire_privilege(SE_SYSTEMTIME_NAME, errp);
     if (error_is_set(errp)) {

qga/qapi-schema.json

@@ -120,17 +120,18 @@
 # This command tries to set guest time to the given value,
 # then sets the Hardware Clock to the current System Time.
 # This will make it easier for a guest to resynchronize
-# without waiting for NTP.
+# without waiting for NTP. If no @time is specified, then
+# the time to set is read from RTC.
 #
-# @time: time of nanoseconds, relative to the Epoch of
+# @time: #optional time of nanoseconds, relative to the Epoch
-#        1970-01-01 in UTC.
+#        of 1970-01-01 in UTC.
 #
 # Returns: Nothing on success.
 #
 # Since: 1.5
 ##
 { 'command': 'guest-set-time',
-  'data': { 'time': 'int' } }
+  'data': { '*time': 'int' } }
 
 ##
 # @GuestAgentCommandInfo: