Re: [Qemu-devel] [PATCH 1/2] qga: add windows implementation for guest-get-time

[mdroth]

On Wed, Mar 13, 2013 at 06:10:30PM +0800, address@hidden wrote:
> From: Lei Li <address@hidden>
> 
> Signed-off-by: Lei Li <address@hidden>
> ---
>  qga/commands-win32.c |   32 ++++++++++++++++++++++++++++++++
>  1 files changed, 32 insertions(+), 0 deletions(-)
> 
> diff --git a/qga/commands-win32.c b/qga/commands-win32.c
> index 7e8ecb3..0a2bb34 100644
> --- a/qga/commands-win32.c
> +++ b/qga/commands-win32.c
> @@ -22,6 +22,12 @@
>  #define SHTDN_REASON_FLAG_PLANNED 0x80000000
>  #endif
> 
> +/* multiple of 100 nanoseconds elapsed between windows baseline
> +   (1/1/1601) and Unix Epoch (1/1/1970), accounting for leap years */
> +#define W32_FT_OFFSET (10000000ULL * 60 * 60 * 24 * \
> +                       (365 * (1970 - 1601) +       \
> +                        (1970 - 1601) / 4 - 3))
> +
>  static void acquire_privilege(const char *name, Error **err)
>  {
>      HANDLE token;
> @@ -108,6 +114,32 @@ void qmp_guest_shutdown(bool has_mode, const char *mode, 
> Error **err)
>      }
>  }
> 
> +int64_t qmp_guest_get_time(Error **errp)
> +{
> +   SYSTEMTIME *ts = g_malloc0(sizeof(SYSTEMTIME));

Don't we need to free this at some point?

> +   int64_t time_ns;
> +   union {
> +       UINT64 ns100;
> +       FILETIME tf;
> +   } time;
> +
> +   GetSystemTime(ts);
> +   if (!ts) {
> +       slog("guest-get-time failed: %d", GetLastError());

GetSystemTime() does not seem to set an error that can be retrieved by
GetLastError().

> +       error_setg_errno(errp, errno, "Failed to get time");
> +       return -1;
> +   }
> +
> +   if (!SystemTimeToFileTime(ts, &time.tf)) {
> +       error_setg_errno(errp, errno, "Failed to convert system time");
> +       return -1;
> +   }
> +
> +   time_ns = (int64_t)((time.ns100 - W32_FT_OFFSET) * 100);

I'm not sure how safe this union stuff is. The documentation suggests that in
some circumstances the low/high fields in FILETIME might be padded for
64-bit alignment and that doing this type of cast could generate an
alignment fault:

http://msdn.microsoft.com/en-us/library/windows/desktop/ms724284(v=vs.85).aspx

or it might, perhaps even worse, just silently report the wrong time.

I think we should just do the math explicitly:

(((tf.dwHighDateTime << 32) | tf.dwLowDateTime) - W32_FT_OFFSET) * 100)

(or something along that line)

> +
> +   return time_ns;
> +}
> +
>  int64_t qmp_guest_file_open(const char *path, bool has_mode, const char 
> *mode, Error **err)
>  {
>      error_set(err, QERR_UNSUPPORTED);
> -- 
> 1.7.7.6
>