From: Eduardo Habkost <ehabkost@redhat.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: "lvivier@redhat.com" <lvivier@redhat.com>,
"thuth@redhat.com" <thuth@redhat.com>,
"mst@redhat.com" <mst@redhat.com>,
"Liu, Jingqi" <jingqi.liu@intel.com>, Tao Xu <tao3.xu@intel.com>,
"Du, Fan" <fan.du@intel.com>,
"mdroth@linux.vnet.ibm.com" <mdroth@linux.vnet.ibm.com>,
"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
"jonathan.cameron@huawei.com" <jonathan.cameron@huawei.com>,
"imammedo@redhat.com" <imammedo@redhat.com>
Subject: Re: [PATCH v14 03/11] tests: Add test for QAPI builtin type time
Date: Tue, 12 Nov 2019 17:15:58 -0300 [thread overview]
Message-ID: <20191112201558.GG3812@habkost.net> (raw)
In-Reply-To: <87lfsqbxnj.fsf@dusky.pond.sub.org>
On Fri, Nov 08, 2019 at 09:05:52AM +0100, Markus Armbruster 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.
>
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. :)
--
Eduardo
next prev parent reply other threads:[~2019-11-12 20:16 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-28 7:52 [PATCH v14 00/11] Build ACPI Heterogeneous Memory Attribute Table (HMAT) Tao Xu
2019-10-28 7:52 ` [PATCH v14 01/11] util/cutils: Add qemu_strtotime_ns() Tao Xu
2019-11-06 19:56 ` Eduardo Habkost
2019-11-07 1:38 ` Tao Xu
2019-10-28 7:52 ` [PATCH v14 02/11] qapi: Add builtin type time Tao Xu
2019-10-28 7:52 ` [PATCH v14 03/11] tests: Add test for QAPI " Tao Xu
2019-11-06 20:53 ` Eduardo Habkost
2019-11-07 6:24 ` Tao Xu
2019-11-07 13:31 ` Eduardo Habkost
2019-11-08 5:25 ` Tao Xu
2019-11-08 8:05 ` Markus Armbruster
2019-11-08 8:41 ` Igor Mammedov
2019-11-11 3:12 ` Tao Xu
2019-11-11 10:02 ` Igor Mammedov
2019-11-12 20:15 ` Eduardo Habkost [this message]
2019-11-13 1:01 ` Tao Xu
2019-11-13 22:06 ` Eduardo Habkost
2019-11-14 0:51 ` Tao Xu
2019-10-28 7:52 ` [PATCH v14 04/11] numa: Extend CLI to provide initiator information for numa nodes Tao Xu
2019-11-06 20:29 ` Eric Blake
2019-11-07 1:51 ` Tao Xu
2019-10-28 7:52 ` [PATCH v14 05/11] numa: Extend CLI to provide memory latency and bandwidth information Tao Xu
2019-10-28 7:52 ` [PATCH v14 06/11] numa: Calculate hmat latency and bandwidth entry list Tao Xu
2019-10-28 7:52 ` [PATCH v14 07/11] numa: Extend CLI to provide memory side cache information Tao Xu
2019-10-28 7:52 ` [PATCH v14 08/11] hmat acpi: Build Memory Proximity Domain Attributes Structure(s) Tao Xu
2019-10-28 7:52 ` [PATCH v14 09/11] hmat acpi: Build System Locality Latency and Bandwidth Information Structure(s) Tao Xu
2019-10-28 7:52 ` [PATCH v14 10/11] hmat acpi: Build Memory Side Cache " Tao Xu
2019-10-28 7:52 ` [PATCH v14 11/11] tests/bios-tables-test: add test cases for ACPI HMAT Tao Xu
2019-10-28 8:39 ` Michael S. Tsirkin
2019-10-28 8:50 ` Tao Xu
2019-10-28 8:36 ` [PATCH v14 00/11] Build ACPI Heterogeneous Memory Attribute Table (HMAT) no-reply
2019-11-06 8:39 ` Tao Xu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20191112201558.GG3812@habkost.net \
--to=ehabkost@redhat.com \
--cc=armbru@redhat.com \
--cc=fan.du@intel.com \
--cc=imammedo@redhat.com \
--cc=jingqi.liu@intel.com \
--cc=jonathan.cameron@huawei.com \
--cc=lvivier@redhat.com \
--cc=mdroth@linux.vnet.ibm.com \
--cc=mst@redhat.com \
--cc=qemu-devel@nongnu.org \
--cc=tao3.xu@intel.com \
--cc=thuth@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).