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

[Anthony Liguori]

Eric Blake <address@hidden> writes:

> On 01/27/2013 11:14 AM, Lei Li wrote:
>> Signed-off-by: Lei Li <address@hidden>
>> ---
>>  include/qapi/qmp/qerror.h |    3 +++
>>  qga/commands-posix.c      |   30 ++++++++++++++++++++++++++++++
>>  qga/qapi-schema.json      |   38 ++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 71 insertions(+), 0 deletions(-)
>> 
>> diff --git a/include/qapi/qmp/qerror.h b/include/qapi/qmp/qerror.h
>> index 6c0a18d..0baf1a4 100644
>> --- a/include/qapi/qmp/qerror.h
>> +++ b/include/qapi/qmp/qerror.h
>> @@ -129,6 +129,9 @@ void assert_no_error(Error *err);
>>  #define QERR_FEATURE_DISABLED \
>>      ERROR_CLASS_GENERIC_ERROR, "The feature '%s' is not enabled"
>>  
>> +#define QERR_GET_TIME_FAILED \
>> +    ERROR_CLASS_GENERIC_ERROR, "Failed to get time"
>> +
>
> These days, you shouldn't be defining a new QERR wrapper.  Instead,...[1]
>
>>  
>> +static TimeInfo *get_host_time(Error **errp)
>
> This name is unusual...[2]
>
>> +{
>> +    int ret;
>> +    qemu_timeval tq;
>> +    TimeInfo *time_info;
>> +
>> +    ret = qemu_gettimeofday(&tq);
>> +    if (ret < 0) {
>> +        error_set_errno(errp, errno, QERR_GET_TIME_FAILED);
>
> [1]...use the right idiom here:
>
> error_setg_errno(errp, errno, "Failed to get time");
>
>> +        return NULL;
>> +    }
>> +
>> +    time_info = g_malloc0(sizeof(TimeInfo));
>> +    time_info->seconds = tq.tv_sec;
>> +    time_info->microseconds = tq.tv_usec;
>
> Is microseconds the right unit to expose through JSON, or are we going
> to wish we had exposed nanoseconds down the road (you can still use
> qemu_gettimeofday() and scale tv_usec by 1000 before assigning to
> time_info->nanoseconds, if we desire the extra granularity).
>
> You aren't setting time_info->utc_offset.  Is that intentional?

Moreover, there's no need to specify microseconds and seconds.  A 64-bit
value for nanoseconds is sufficient.  A signed value can represent
1678 -> 2262.  If anyone is using qemu-ga in 2262, they'll have to
introduce a new command for their quantum emulators :-)

Setting time independent of date is a bit silly because time cannot be
interpreted correctly without a date.

A single value of nanoseconds since the epoch (interpreted as UTC/GMT
time) is really the best strategy.  The host shouldn't be involved in
guest time zones.  In a Cloud environment, it's pretty normal to have
different guests using different time zones.

Regards,

Anthony Liguori

>
>> +
>> +    return time_info;
>> +}
>> +
>> +struct TimeInfo *qmp_guest_get_time(Error **errp)
>> +{
>> +    TimeInfo *time_info = get_host_time(errp);
>
> [2]...given that it is only ever called from qmp_guest_get_time.
>
>> +
>> +    if (!time_info) {
>> +        return NULL;
>> +    }
>
> These three lines can be deleted,
>
>> +
>> +    return time_info;
>
> since this line will do the same thing if you let NULL time_info get
> this far.
>
>> +++ b/qga/qapi-schema.json
>> @@ -83,6 +83,44 @@
>>  { 'command': 'guest-ping' }
>>  
>>  ##
>> +# @TimeInfo
>> +#
>> +# Time Information. It is relative to the Epoch of 1970-01-01.
>> +#
>> +# @seconds: "seconds" time unit.
>> +#
>> +# @microseconds: "microseconds" time unit.
>> +#
>> +# @utc-offset: Information about utc offset. Represented as:
>> +#              ±[mmmm] based at a minimum on minutes, with
>
> s/based at a minimum on//
>
> This still doesn't state whether two hours east of UTC is represented as
> 120 or as 0200.
>
>> +#              negative values are west and positive values
>> +#              are east of UTC. The bounds of @utc-offset is
>> +#              at most 24 hours away from UTC.
>> +#
>> +# Since: 1.4
>> +##
>> +{ 'type': 'TimeInfo',
>> +  'data': { 'seconds': 'int', 'microseconds': 'int',
>> +            'utc-offset': 'int' } }
>> +
>> +##
>> +# @guest-get-time:
>> +#
>> +# Get the information about host time in UTC and the
>
> s/host/guest/
>
>> +# UTC offset.
>> +#
>> +# This command tries to get the host time which is
>> +# presumably correct, since we need to be able to
>> +# resynchronize guest clock to host's in guest.
>
> This sentence doesn't make sense.  Better would be:
>
> Get the guest's notion of the current time.
>
>> +#
>> +# Returns: @TimeInfo on success.
>> +#
>> +# Since 1.4
>> +##
>> +{ 'command': 'guest-get-time',
>> +  'returns': 'TimeInfo' }
>> +
>> +##
>>  # @GuestAgentCommandInfo:
>>  #
>>  # Information about guest agent commands.
>> 
>
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org