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Re: [Qemu-ppc] [PATCHv2 8/8] pseries: Export RTC time via QOM

From: David Gibson
Subject: Re: [Qemu-ppc] [PATCHv2 8/8] pseries: Export RTC time via QOM
Date: Wed, 28 Jan 2015 16:28:24 +1100
User-agent: Mutt/1.5.23 (2014-03-12)

On Tue, Dec 23, 2014 at 07:33:30AM +0100, Alexander Graf wrote:
> > Am 23.12.2014 um 04:14 schrieb David Gibson <address@hidden>:
> > 
> >> On Tue, Dec 23, 2014 at 01:26:58AM +0100, Alexander Graf wrote:
> >> 
> >> 
> >>> On 23.12.14 01:17, David Gibson wrote:
> >>> On x86, the guest's RTC can be read with QMP, either from the RTC device's
> >>> "date" property or via the "rtc-time" property on the machine (which is an
> >>> alias to the former).  This is set up in the mc146818rtc driver, and
> >>> doesn't work on other targets.
> >>> 
> >>> This patch adds a similar "date" property to the pseries machine's RTAS 
> >>> RTC
> >>> and adds a compatible alias to the machine.
> >>> 
> >>> Signed-off-by: David Gibson <address@hidden>
> >> 
> >> Very nice, can we somehow get rid of an exported spapr_rtc_read() with
> >> this as well?
> > 
> > Uhhh.. we could, but I really don't like the idea.
> > 
> > It seems perverse to encode the current time into JSON, just so we can
> > decode it into a struct tm again in the events code.
> I don't understand - what does JSON have to do with it?

My misunderstanding, it doesn't actually deal with JSON here.  But if
I'm following how this would work, it would be a way of getting a
struct tm from point A to point B, by:
  1. building a temporary visitor object (object_property_get_qobject)
  2. finding and calling the general property getter callback
  3. Making multiple callbacks into the visitor with pieces of point
     A's struct tm (property_get_tm)
  4. In those visitor callbacks building an associative array
     (qmp_output_start_struct), then filling it with object wrappers
     around integers (qmp_output_type_int)
  5. Digging through the nested qobject thus constructed to extract
     the various ints and put them into point B's struct tm
     (hypothetical object_property_get_tm)
  6. Cleaning up the various temporaries created during the process

Seriously, w. t. f.!?

Maybe there's a simpler way of doing this via the object property
mechanism, but it's sure as hell not obvious to me.

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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