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

[Eduardo Habkost]

On Wed, Nov 13, 2019 at 09:01:29AM +0800, Tao Xu wrote:
> On 11/13/2019 4:15 AM, Eduardo Habkost wrote:
> > On Fri, Nov 08, 2019 at 09:05:52AM +0100, Markus Armbruster 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.
> > > 
> > 
> > This sounds like the right thing to do, if user input is an
> > integer and the code in the other end is consuming an integer.
> > 
> > 
> > > As far as I remember, the only problem we've ever had with the 53 bits
> > > limit is developer confusion :)
> > > 
> > 
> > Developer confusion, I can deal with.  However, exposing this
> > behavior on external interfaces is a bug to me.
> > 
> > I don't know how serious the bug is because I don't know which
> > interfaces are affected by it.  Do we have a list?
> > 
> > > Patches welcome.
> > 
> > My first goal is to get the maintainers of that code to recognize
> > it as a bug.  Then I hope this will motivate somebody else to fix
> > it.  :)
> > 
> 
> Hi Eduardo,
> 
> If it is a bug, could the fix patch merged during rc1-rc3? Because I made 2
> patches, and I want to submit before HMAT (HMAT patches is big, so submit
> together may be slow).

Even if I convince other maintainers it is a bug, I don't think
it is serious enough to require a fix in QEMU 4.2.  I suggest
finishing the ongoing HMAT work first, and worry about this issue
later.

Or, if you really prefer to address it before HMAT, it's OK to
make the next version of the HMAT series depend on a series
that's not merged yet.  Just make this explicit in the series
cover letter (publishing a git branch to help review and testing
is also appreciated).

-- 
Eduardo