Re: [Qemu-devel] [RFC] Can we convert UTC time to local time in Qemu

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, May 20, 2015 at 06:29:55AM +0000, Gonglei (Arei) wrote:
> Hi,
> 
> At present, Qemu use g_time_val_to_iso8601() to get the UTC added to error_report()
> (commit 5e2ac5191), TBH this way is very simply, we just need invoke standard glib functions to
> complete the job. 
> 
> But in the cloud computing and data center scenarios, there are many
> Other open source components, such as kernel, libvirt, openstack which are all using local time
> to record the message logs, only Qemu is using the UTC time. When we want to find a error
> message of Qemu, we should convert the UTC time to local time manually, and unfortunately
> different countries have different local time, what a trouble thing the converting is.
> 
> So, my question is: Can we convert the UTC time to local time in Qemu?
> 
> Any thoughts? Thanks.

Actually, libvirt uses UTC for logging almost exclusively for quite a
while, because AFAIK, there is no async signal safe way to convert to
localtime in POSIX / glibc

commit 3ec128989606278635a7c5dfbeee959692d12e15
Author: Daniel P. Berrange <berrange@redhat.com>
Date:   Tue Nov 29 12:11:01 2011 +0000

    Add internal APIs for dealing with time
    
    The logging APIs need to be able to generate formatted timestamps
    using only async signal safe functions. This rules out using
    gmtime/localtime/malloc/gettimeday(!) and much more.
    
    Introduce a new internal API which is async signal safe.
    
      virTimeMillisNowRaw replacement for gettimeofday. Uses clock_gettime
                          where available, otherwise falls back to the unsafe
                          gettimeofday
    
      virTimeFieldsNowRaw  replacements for gmtime(), convert a timestamp
      virTimeFieldsThenRaw into a broken out set of fields. No localtime()
                           replacement is provided, because converting to
                           local time is not practical with only async signal
                           safe APIs.
    
      virTimeStringNowRaw  replacements for strftime() which print a timestamp
      virTimeStringThenRaw into a string, using a pre-determined format, with
                           a fixed size buffer (VIR_TIME_STRING_BUFLEN)
    
    For each of these there is also a version without the Raw postfix
    which raises a full libvirt error. These versions are not async
    signal safe

Regards,
Daniel
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