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

[Tao Xu]

On 11/8/2019 4:41 PM, Igor Mammedov wrote:
> On Fri, 08 Nov 2019 09:05:52 +0100
> Markus Armbruster <address@hidden> wrote:
>
> > Tao Xu <address@hidden> 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 <address@hidden>
> > > > > > > ---
> > > > > > [...]
> > > > > > > +    /* 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.