Re: [PATCH RFC] qga: fence guest-set-time if hwclock not available

[Laszlo Ersek <lersek@redhat.com>]

On 11/28/19 13:45, Daniel P. Berrangé wrote:
> On Thu, Nov 28, 2019 at 01:36:58PM +0100, Cornelia Huck wrote:
>> The Posix implementation of guest-set-time invokes hwclock to
>> set/retrieve the time to/from the hardware clock. If hwclock
>> is not available, the user is currently informed that "hwclock
>> failed to set hardware clock to system time", which is quite
>> misleading. This may happen e.g. on s390x, which has a different
>> timekeeping concept anyway.
>>
>> Let's check for the availability of the hwclock command and
>> return QERR_UNSUPPORTED for guest-set-time if it is not available.
>>
>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
>> ---
>>
>> Not sure if that is the correct approach, but the current error
>> message is really quite confusing.
> 
> I guess the alternative is to just #ifndef __s390x__ the whole
> impl of the qmp_guest_set_time  method, but I don't have a
> strong opinion on which is best.
> 
>>
>> Gave it a quick test with an s390x and an x86_64 guest; invoking
>> 'virsh domtime <value>' now fails with 'not currently supported'
>> on s390x and continues to work as before on x86_64.
>>
>> ---
>>  qga/commands-posix.c | 20 +++++++++++++++++++-
>>  1 file changed, 19 insertions(+), 1 deletion(-)
>>
>> diff --git a/qga/commands-posix.c b/qga/commands-posix.c
>> index 1c1a165daed8..bd298a38b716 100644
>> --- a/qga/commands-posix.c
>> +++ b/qga/commands-posix.c
>> @@ -149,6 +149,13 @@ int64_t qmp_guest_get_time(Error **errp)
>>     return tq.tv_sec * 1000000000LL + tq.tv_usec * 1000;
>>  }
>>  
>> +static int check_hwclock_available(const char *path)
>> +{
>> +    struct stat st;
>> +
>> +    return (stat(path, &st) < 0) ? 0 : 1;
>> +}
>> +
>>  void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
>>  {
>>      int ret;
>> @@ -156,6 +163,17 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
>>      pid_t pid;
>>      Error *local_err = NULL;
>>      struct timeval tv;
>> +    const char *hwclock_path = "/sbin/hwclock";

slight style / semantics improvement could be

  static const char hwclock_path[] = "/sbin/hwclock";

Thanks
Laszlo

>> +    static int hwclock_available = -1;
>> +
>> +    if (hwclock_available < 0) {
>> +        hwclock_available = check_hwclock_available(hwclock_path);
> 
> Could do this inline with:
> 
>     hwclock_available = (access(hwclock_available, X_OK) == 0);
> 
> getting a slightly better result as this check for it being
> executable as well as existing.
> 
>> +    }
>> +
>> +    if (!hwclock_available) {
>> +        error_setg(errp, QERR_UNSUPPORTED);
>> +        return;
>> +    }
>>  
>>      /* If user has passed a time, validate and set it. */
>>      if (has_time) {
>> @@ -195,7 +213,7 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
>>  
>>          /* 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",
>> +        execle(hwclock_path, "hwclock", has_time ? "-w" : "-s",
>>                 NULL, environ);
>>          _exit(EXIT_FAILURE);
>>      } else if (pid < 0) {
>> -- 
>> 2.21.0
>>
>>
> 
> Regards,
> Daniel
>