[Qemu-devel] [PATCH] qga: Don't require 'time' argument in guest-set-time command

[Michal Privoznik]

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 <address@hidden>
---

Notes:
    I wasn't sure about enforcing Windows to fetch time from RTC. But I've found
    this site [1] which seems to presume that GetSystemTime() reads RTC. 
Although
    within the page you'll find GetSystemTimeAsFileTime() which is the same 
except
    returned time is in different format. That's all.
    
    1: 
http://nadeausoftware.com/articles/2012/04/c_c_tip_how_measure_elapsed_real_time_benchmarking

 qga/commands-posix.c | 40 ++++++++++++++++++++++++----------------
 qga/commands-win32.c | 33 ++++++++++++++++++++++-----------
 qga/qapi-schema.json |  9 +++++----
 3 files changed, 51 insertions(+), 31 deletions(-)

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 8100bee..6d36ab5 100644
--- a/qga/commands-posix.c
+++ b/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;
@@ -150,22 +150,28 @@ void qmp_guest_set_time(int64_t time_ns, Error **errp)
     Error *local_err = NULL;
     struct timeval tv;
 
-    /* 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);
-        return;
+    /* 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);
+            return;
+        }
+
+        tv.tv_sec = time_ns / 1000000000;
+        tv.tv_usec = (time_ns % 1000000000) / 1000;
+
+        ret = settimeofday(&tv, NULL);
+        if (ret < 0) {
+            error_setg_errno(errp, errno, "Failed to set time to guest");
+            return;
+        }
     }
 
-    tv.tv_sec = time_ns / 1000000000;
-    tv.tv_usec = (time_ns % 1000000000) / 1000;
-
-    ret = settimeofday(&tv, NULL);
-    if (ret < 0) {
-        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 +179,9 @@ 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");
diff --git a/qga/commands-win32.c b/qga/commands-win32.c
index a6a0af2..9ce89ce 100644
--- a/qga/commands-win32.c
+++ b/qga/commands-win32.c
@@ -370,25 +370,36 @@ 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 (time_ns < 0 || time_ns / 100 > INT64_MAX - W32_FT_OFFSET) {
-        error_setg(errp, "Time %" PRId64 "is invalid", time_ns);
-        return;
-    }
+    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;
+        }
 
-    time = time_ns / 100 + W32_FT_OFFSET;
+        time = time_ns / 100 + W32_FT_OFFSET;
 
-    tf.dwLowDateTime = (DWORD) time;
-    tf.dwHighDateTime = (DWORD) (time >> 32);
+        tf.dwLowDateTime = (DWORD) time;
+        tf.dwHighDateTime = (DWORD) (time >> 32);
 
-    if (!FileTimeToSystemTime(&tf, &ts)) {
-        error_setg(errp, "Failed to convert system time %d", 
(int)GetLastError());
-        return;
+        if (!FileTimeToSystemTime(&tf, &ts)) {
+            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);
diff --git a/qga/qapi-schema.json b/qga/qapi-schema.json
index 245f968..80edca1 100644
--- a/qga/qapi-schema.json
+++ b/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
-#        1970-01-01 in UTC.
+# @time: #optional time of nanoseconds, relative to the Epoch
+#        of 1970-01-01 in UTC.
 #
 # Returns: Nothing on success.
 #
 # Since: 1.5
 ##
 { 'command': 'guest-set-time',
-  'data': { 'time': 'int' } }
+  'data': { '*time': 'int' } }
 
 ##
 # @GuestAgentCommandInfo:
-- 
1.8.5.2