Re: [PATCH v14 03/11] tests: Add test for QAPI builtin type time

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 11 Nov 2019 11:12:28 +0800
Tao Xu <tao3.xu@intel.com> wrote:

> On 11/8/2019 4:41 PM, Igor Mammedov wrote:
> > On Fri, 08 Nov 2019 09:05:52 +0100
> > Markus Armbruster <armbru@redhat.com> wrote:
> >   
> >> Tao Xu <tao3.xu@intel.com> writes:
> >>  
> >>> On 11/7/2019 9:31 PM, Eduardo Habkost wrote:  
> >>>> On Thu, Nov 07, 2019 at 02:24:52PM +0800, Tao Xu wrote:  
> >>>>> On 11/7/2019 4:53 AM, Eduardo Habkost wrote:  
> >>>>>> On Mon, Oct 28, 2019 at 03:52:12PM +0800, Tao Xu wrote:  
> >>>>>>> Add tests for time input such as zero, around limit of precision,
> >>>>>>> signed upper limit, actual upper limit, beyond limits, time suffixes,
> >>>>>>> and etc.
> >>>>>>>
> >>>>>>> Signed-off-by: Tao Xu <tao3.xu@intel.com>
> >>>>>>> ---  
> >>>>>> [...]  
> >>>>>>> +    /* Close to signed upper limit 0x7ffffffffffffc00 (53 msbs set) */
> >>>>>>> +    qdict = keyval_parse("time1=9223372036854774784," /* 7ffffffffffffc00 */
> >>>>>>> +                         "time2=9223372036854775295", /* 7ffffffffffffdff */
> >>>>>>> +                         NULL, &error_abort);
> >>>>>>> +    v = qobject_input_visitor_new_keyval(QOBJECT(qdict));
> >>>>>>> +    qobject_unref(qdict);
> >>>>>>> +    visit_start_struct(v, NULL, NULL, 0, &error_abort);
> >>>>>>> +    visit_type_time(v, "time1", &time, &error_abort);
> >>>>>>> +    g_assert_cmphex(time, ==, 0x7ffffffffffffc00);
> >>>>>>> +    visit_type_time(v, "time2", &time, &error_abort);
> >>>>>>> +    g_assert_cmphex(time, ==, 0x7ffffffffffffc00);  
> >>>>>>
> >>>>>> I'm confused by this test case and the one below[1].  Are these
> >>>>>> known bugs?  Shouldn't we document them as known bugs?  
> >>>>>
> >>>>> Because do_strtosz() or do_strtomul() actually parse with strtod(), so the
> >>>>> precision is 53 bits, so in these cases, 7ffffffffffffdff and
> >>>>> fffffffffffffbff are rounded.  
> >>>>
> >>>> My questions remain: why isn't this being treated like a bug?
> >>>>     
> >>> Hi Markus,
> >>>
> >>> I am confused about the code here too. Because in do_strtosz(), the
> >>> upper limit is
> >>>
> >>> val * mul >= 0xfffffffffffffc00
> >>>
> >>> So some data near 53 bit may be rounded. Is there a bug?  
> >>
> >> No, but the design is surprising, and the functions lack written
> >> contracts, except for the do_strtosz() helper, which has one that sucks.
> >>
> >> qemu_strtosz() & friends are designed to accept fraction * unit
> >> multiplier.  Example: 1.5M means 1.5 * 1024 * 1024 with qemu_strtosz()
> >> and qemu_strtosz_MiB(), and 1.5 * 1000 * 1000 with
> >> qemu_strtosz_metric().  Whether supporting fractions is a good idea is
> >> debatable, but it's what we've got.
> >>
> >> The implementation limits the numeric part to the precision of double,
> >> i.e. 53 bits.  "8PiB should be enough for anybody."
> >>
> >> Switching it from double to long double raises the limit to the
> >> precision of long double.  At least 64 bit on common hosts, but hosts
> >> exist where it's the same 53 bits.  Do we support any such hosts?  If
> >> yes, then we'd make the precision depend on the host, which feels like a
> >> bad idea.
> >>
> >> A possible alternative is to parse the numeric part both as a double and
> >> as a 64 bit unsigned integer, then use whatever consumes more
> >> characters.  This enables providing full 64 bits unless you actually use
> >> a fraction.
> >>
> >> As far as I remember, the only problem we've ever had with the 53 bits
> >> limit is developer confusion :)  
> > 
> > On CLI, we could (a)use full 64bit (-1) lat/bw to mark unreachable nodes.
> > Also it would be more consistent for both QMP and CLI to be able
> > handle the same range. This way what was configured over QMP could be
> > also configured using CLI.
> >   
> 
> OK. I will add a new patch to do this. Next version we will submit
> separated patches for QAPI builtin type changes.

Since you've got rid of magic handling from CLI parsing, it's not
must have feauture but it would be nice to have and probably could
be done on top of HMAT patches.