From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,T_HK_NAME_DR,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0D8A4C04AA7 for ; Mon, 13 May 2019 12:32:26 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C7C5420578 for ; Mon, 13 May 2019 12:32:25 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C7C5420578 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([127.0.0.1]:56535 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hQA80-0000Tw-QX for qemu-devel@archiver.kernel.org; Mon, 13 May 2019 08:32:24 -0400 Received: from eggs.gnu.org ([209.51.188.92]:45218) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hQA6I-0007bv-RL for qemu-devel@nongnu.org; Mon, 13 May 2019 08:30:40 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hQA5T-0008Hr-8p for qemu-devel@nongnu.org; Mon, 13 May 2019 08:29:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41422) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hQA5P-0008G3-W9 for qemu-devel@nongnu.org; Mon, 13 May 2019 08:29:44 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 077703079B9D for ; Mon, 13 May 2019 12:29:41 +0000 (UTC) Received: from work-vm (ovpn-117-195.ams2.redhat.com [10.36.117.195]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 716C010027BF; Mon, 13 May 2019 12:29:36 +0000 (UTC) Date: Mon, 13 May 2019 13:29:34 +0100 From: "Dr. David Alan Gilbert" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Message-ID: <20190513122933.GC2786@work-vm> References: <20190430131919.GN6818@redhat.com> <20190430144546.GA3065@work-vm> <20190430150556.GA2423@redhat.com> <87sgtqejn9.fsf@dusky.pond.sub.org> <20190507093954.GG27205@redhat.com> <875zql3ylk.fsf@dusky.pond.sub.org> <20190513120856.GH15029@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20190513120856.GH15029@redhat.com> User-Agent: Mutt/1.11.4 (2019-03-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Mon, 13 May 2019 12:29:41 +0000 (UTC) Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] QMP; unsigned 64-bit ints; JSON standards compliance X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: libvir-list@redhat.com, =?iso-8859-1?B?SuFu?= Tomko , Markus Armbruster , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Daniel P. Berrang=E9 (berrange@redhat.com) wrote: > On Wed, May 08, 2019 at 02:44:07PM +0200, Markus Armbruster wrote: > > Daniel P. Berrang=E9 writes: > >=20 > > > On Tue, May 07, 2019 at 10:47:06AM +0200, Markus Armbruster wrote: > > > > > >> >> > I can think of some options: > > >> >> >=20 > > >> >> > 1. Encode unsigned 64-bit integers as signed 64-bit integer= s. > > >> >> >=20 > > >> >> > This follows the example that most C libraries map JSON = ints > > >> >> > to 'long long int'. This is still relying on undefined > > >> >> > behaviour as apps don't need to support > 2^53-1. > > >> >> >=20 > > >> >> > Apps would need to cast back to 'unsigned long long' for > > >> >> > those QMP fields they know are supposed to be unsigned. > > >>=20 > > >> Ugly. It's also what we did until v2.10, August 2017. QMP's inpu= t > > >> direction still does it, for backward compatibility. > > >>=20 > > >> >> >=20 > > >> >> >=20 > > >> >> > 2. Encode all 64-bit integers as a pair of 32-bit integers. > > >> >> > =20 > > >> >> > This is fully compliant with the JSON spec as each half > > >> >> > is fully within the declared limits. App has to split or > > >> >> > assemble the 2 pieces from/to a signed/unsigned 64-bit > > >> >> > int as needed. > > >>=20 > > >> Differently ugly. > > >>=20 > > >> >> >=20 > > >> >> >=20 > > >> >> > 3. Encode all 64-bit integers as strings > > >> >> >=20 > > >> >> > The application has todo all parsing/formatting client > > >> >> > side. > > >>=20 > > >> Yet another ugly. > > >>=20 > > >> >> >=20 > > >> >> >=20 > > >> >> > None of these changes are backwards compatible, so I doubt we= could make > > >> >> > the change transparently in QMP. Instead we would have to ha= ve a > > >> >> > QMP greeting message capability where the client can request = enablement > > >> >> > of the enhanced integer handling. > > >>=20 > > >> We might be able to do option 1 without capability negotiation. v= 2.10's > > >> change from option 1 to what we have now produced zero complaints. > > >>=20 > > >> On the other hand, we made that change for a reason, so we may wan= t a > > >> "send large integers as negative integers" capability regardless. > > >>=20 > > >> >> >=20 > > >> >> > Any of the three options above would likely work for libvirt,= but I > > >> >> > would have a slight preference for either 2 or 3, so that we = become > > >> >> > 100% standards compliant. > > >>=20 > > >> There's no such thing. You mean "we maximize interoperability wit= h > > >> common implementations of JSON". > > > > > > s/common/any/ > >=20 > > info: error correction applied, future applications will be silent ;-= P > >=20 > > >> Let's talk implementation for a bit. > > >>=20 > > >> Encoding and decoding integers in funny ways should be fairly easy= in > > >> the QObject visitors. The generated QMP marshallers all use them. > > >> Trouble is a few commands still bypass the generated marshallers, = and > > >> mess with the QObject themselves: > > >>=20 > > >> * query-qmp-schema: minor hack explained in qmp_query_qmp_schema()= 's > > >> comment. Should be harmless. > > >>=20 > > >> * netdev_add: not QAPIfied. Eric's patches to QAPIfy it got stuck > > >> because they reject some abuses like passing numbers and bools a= s > > >> strings. > > >>=20 > > >> * device_add: not QAPIfied. We're not sure QAPIfication is feasib= le. > > >>=20 > > >> netdev_add and device_add both use qemu_opts_from_qdict(). Perhap= s we > > >> could hack that to mirror what the QObject visitor do. > > >>=20 > > >> Else, we might have to do it in the JSON parser. Should be possib= le, > > >> but I'd rather not. > > >>=20 > > >> >> My preference would be 3 with the strings defined as being > > >> >> %x lower case hex formated with a 0x prefix and no longer than = 18 characters > > >> >> ("0x" + 16 nybbles). Zero padding allowed but not required. > > >> >> It's readable and unambiguous when dealing with addresses; I do= n't want > > >> >> to have to start decoding (2) by hand when debugging. > > >> > > > >> > Yep, that's a good point about readability. > > >>=20 > > >> QMP sending all integers in decimal is inconvenient for some value= s, > > >> such as addresses. QMP sending all (large) integers in hexadecima= l > > >> would be inconvenient for other values. > > >>=20 > > >> Let's keep it simple & stupid. If you want sophistication, JSON i= s the > > >> wrong choice. > > >>=20 > > >>=20 > > >> Option 1 feels simplest. > > > > > > But will still fail with any JSON impl that uses double precision f= loating > > > point for integers as it will loose precision. > > > > > >> Option 2 feels ugliest. Less simple, more interoperable than opti= on 1. > > > > > > If we assume any JSON impl can do 32-bit integers without loss of > > > precision, then I think we can say it is guaranteed portable, but > > > it is certainly horrible / ugly. > > > > > >> Option 3 is like option 2, just not quite as ugly. > > > > > > I think option 3 can be guaranteed to be loss-less with /any/ JSON = impl > > > that exists, since you're delegating all string -> int conversion t= o > > > the application code taking the JSON parser/formatter out of the eq= uation. > >=20 > > Double-checking: do you propose to encode *all* numbers as strings, o= r > > just certain "problematic" numbers? > >=20 > > If the latter, I guess your idea of "problematic" is "not representab= le > > exactly as double precision floating-point". >=20 > We have a few options >=20 > 1. Use string format for values > 2^53-1, int format below that > 2. Use string format for all fields which are 64-bit ints whether > signed or unsigned > 3. Use string format for all fields which are integers, even 32-bit > ones >=20 > I would probably suggest option 2. It would make the QEMU impl quite > easy IIUC, we we'd just change the QAPI visitor's impl for the int64 > and uint64 fields to use string format (when the right capability is > negotiated by QMP). >=20 > I include 3 only for completeness - I don't think there's a hugely > compelling reason to mess with 32-bit ints. What about when the size is architecture dependent? > Option 1 is the bare minimum needed to ensure precision, but to me > it feels a bit dirty to say a given field will have different encoding > depending on the value. If apps need to deal with string encoding, they > might as well just use it for all values in a given field. Yeh, 1 is horrid; it's too easy to miss a case which forgot to handle the 2^53-1 because we hadn't forced a large value down that check. Dave > > > I guess I'd have a preference for option 3 given that it has better > > > interoperability > >=20 > > If we add a QMP capability for interoperability with JSON > > implementations that set limits on range and precision that are > > incompatible with the ones QMP sets, one could argue we effectively p= ay > > the price for option 3, so we should take it for its benefits. > >=20 > > Option 1 without a QMP capability merely reverts the change we made i= n > > 2.10. We can do that if we think it's sufficient. > >=20 > > You expressed a strong preference for maximizing interoperability (vi= a > > option 3). Acknowledged. However, I care a lot more about issues we > > know we have than about issues somebody might have. > >=20 > > You mentioned the libvirt's switch to Jansson you had to abort due to > > QMP sending numbers Jansson refuses to parse. That's the kind of > > non-hypothetical issue that can make me mess with the QMP language. > >=20 > > You wrote Jansson "raises a fatal parse error for unsigned 64-bit val= ues > > above 2^63-1". Does that mean it rejects 9223372036854775808, but > > accepts 9223372036854775808.0 (with loss of precision)? >=20 > If it sees a '.' in the number, then it call strtod() and checks for > the overflow conditions. >=20 > If it doesn't see a '.' in the number then it calls strtoll and checks > for the overflow conditions. >=20 > So to answer you question, yes, it looks like it will reject > 9223372036854775808 and accept 9223372036854775808.0 with loss of > precision. >=20 > Regards, > Daniel > --=20 > |: https://berrange.com -o- https://www.flickr.com/photos/dberr= ange :| > |: https://libvirt.org -o- https://fstop138.berrange= .com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberr= ange :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK