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=-2.4 required=3.0 tests=FROM_EXCESS_BASE64, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,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 94D00C04AA7 for ; Mon, 13 May 2019 12:37:05 +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 56E4E2084E for ; Mon, 13 May 2019 12:37:05 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 56E4E2084E 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]:56602 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hQACV-0002ct-Sl for qemu-devel@archiver.kernel.org; Mon, 13 May 2019 08:37:03 -0400 Received: from eggs.gnu.org ([209.51.188.92]:46290) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hQABP-0001xv-QD for qemu-devel@nongnu.org; Mon, 13 May 2019 08:35:57 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hQABO-0002vO-6Z for qemu-devel@nongnu.org; Mon, 13 May 2019 08:35:55 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57236) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hQABN-0002qj-6Y for qemu-devel@nongnu.org; Mon, 13 May 2019 08:35:53 -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 5A6EC307D854 for ; Mon, 13 May 2019 12:35:48 +0000 (UTC) Received: from redhat.com (unknown [10.42.22.189]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B38AD1001E79; Mon, 13 May 2019 12:35:44 +0000 (UTC) Date: Mon, 13 May 2019 13:35:42 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: "Dr. David Alan Gilbert" Message-ID: <20190513123542.GJ15029@redhat.com> 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> <20190513122933.GC2786@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190513122933.GC2786@work-vm> 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 [10.5.110.48]); Mon, 13 May 2019 12:35:48 +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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: libvir-list@redhat.com, =?utf-8?Q?J=C3=A1n?= Tomko , Markus Armbruster , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, May 13, 2019 at 01:29:34PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrang=C3=A9 (berrange@redhat.com) wrote: > > On Wed, May 08, 2019 at 02:44:07PM +0200, Markus Armbruster wrote: > > > Daniel P. Berrang=C3=A9 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 integ= ers. > > > >> >> >=20 > > > >> >> > This follows the example that most C libraries map JSO= N 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' f= or > > > >> >> > 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 in= put > > > >> direction still does it, for backward compatibility. > > > >>=20 > > > >> >> >=20 > > > >> >> >=20 > > > >> >> > 2. Encode all 64-bit integers as a pair of 32-bit integer= s. > > > >> >> > =20 > > > >> >> > This is fully compliant with the JSON spec as each hal= f > > > >> >> > 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 = have a > > > >> >> > QMP greeting message capability where the client can reques= t enablement > > > >> >> > of the enhanced integer handling. > > > >>=20 > > > >> We might be able to do option 1 without capability negotiation. = v2.10's > > > >> change from option 1 to what we have now produced zero complaint= s. > > > >>=20 > > > >> On the other hand, we made that change for a reason, so we may w= ant a > > > >> "send large integers as negative integers" capability regardless= . > > > >>=20 > > > >> >> >=20 > > > >> >> > Any of the three options above would likely work for libvir= t, but I > > > >> >> > would have a slight preference for either 2 or 3, so that w= e become > > > >> >> > 100% standards compliant. > > > >>=20 > > > >> There's no such thing. You mean "we maximize interoperability w= ith > > > >> 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 ea= sy in > > > >> the QObject visitors. The generated QMP marshallers all use the= m. > > > >> 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 stu= ck > > > >> because they reject some abuses like passing numbers and bools= as > > > >> strings. > > > >>=20 > > > >> * device_add: not QAPIfied. We're not sure QAPIfication is feas= ible. > > > >>=20 > > > >> netdev_add and device_add both use qemu_opts_from_qdict(). Perh= aps 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 poss= ible, > > > >> 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 tha= n 18 characters > > > >> >> ("0x" + 16 nybbles). Zero padding allowed but not required. > > > >> >> It's readable and unambiguous when dealing with addresses; I = don'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 val= ues, > > > >> such as addresses. QMP sending all (large) integers in hexadeci= mal > > > >> would be inconvenient for other values. > > > >>=20 > > > >> Let's keep it simple & stupid. If you want sophistication, JSON= is the > > > >> wrong choice. > > > >>=20 > > > >>=20 > > > >> Option 1 feels simplest. > > > > > > > > But will still fail with any JSON impl that uses double precision= floating > > > > point for integers as it will loose precision. > > > > > > > >> Option 2 feels ugliest. Less simple, more interoperable than op= tion 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/ JSO= N impl > > > > that exists, since you're delegating all string -> int conversion= to > > > > the application code taking the JSON parser/formatter out of the = equation. > > >=20 > > > Double-checking: do you propose to encode *all* numbers as strings,= or > > > just certain "problematic" numbers? > > >=20 > > > If the latter, I guess your idea of "problematic" is "not represent= able > > > 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. >=20 > What about when the size is architecture dependent? The QAPI visitor for 'int' uses an 'int64_t' parameters, so I think that will want to be string encoded, as if it was a 64-bit int, even if built on a 32-bit platform. >=20 > > 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 encodin= g > > depending on the value. If apps need to deal with string encoding, th= ey > > might as well just use it for all values in a given field. >=20 > 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. Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberran= ge :| |: https://libvirt.org -o- https://fstop138.berrange.c= om :| |: https://entangle-photo.org -o- https://www.instagram.com/dberran= ge :|