Re: [libvirt] [Qemu-devel] QMP; unsigned 64-bit ints; JSON standards compliance

* Daniel P. Berrangé (berrange(a)redhat.com) wrote: > On Wed, May 08, 2019 at 02:44:07PM +0200, Markus Armbruster wrote: > > Daniel P. Berrangé <berrange(a)redhat.com&gt; writes: > > > > > On Tue, May 07, 2019 at 10:47:06AM +0200, Markus Armbruster wrote: > > > > > >> >> > I can think of some options: > > >> >> > > > >> >> > 1. Encode unsigned 64-bit integers as signed 64-bit integers. > > >> >> > > > >> >> > 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. > > >> >> > > > >> >> > Apps would need to cast back to 'unsigned long long' for > > >> >> > those QMP fields they know are supposed to be unsigned. > > >> > > >> Ugly. It's also what we did until v2.10, August 2017. QMP's input > > >> direction still does it, for backward compatibility. > > >> > > >> >> > > > >> >> > > > >> >> > 2. Encode all 64-bit integers as a pair of 32-bit integers. > > >> >> > > > >> >> > 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. > > >> > > >> Differently ugly. > > >> > > >> >> > > > >> >> > > > >> >> > 3. Encode all 64-bit integers as strings > > >> >> > > > >> >> > The application has todo all parsing/formatting client > > >> >> > side. > > >> > > >> Yet another ugly. > > >> > > >> >> > > > >> >> > > > >> >> > 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 request enablement > > >> >> > of the enhanced integer handling. > > >> > > >> 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 complaints. > > >> > > >> On the other hand, we made that change for a reason, so we may want a > > >> "send large integers as negative integers" capability regardless. > > >> > > >> >> > > > >> >> > 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. > > >> > > >> There's no such thing. You mean "we maximize interoperability with > > >> common implementations of JSON". > > > > > > s/common/any/ > > > > info: error correction applied, future applications will be silent ;-P > > > > >> Let's talk implementation for a bit. > > >> > > >> 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: > > >> > > >> * query-qmp-schema: minor hack explained in qmp_query_qmp_schema()'s > > >> comment. Should be harmless. > > >> > > >> * netdev_add: not QAPIfied. Eric's patches to QAPIfy it got stuck > > >> because they reject some abuses like passing numbers and bools as > > >> strings. > > >> > > >> * device_add: not QAPIfied. We're not sure QAPIfication is feasible. > > >> > > >> netdev_add and device_add both use qemu_opts_from_qdict(). Perhaps we > > >> could hack that to mirror what the QObject visitor do. > > >> > > >> Else, we might have to do it in the JSON parser. Should be possible, > > >> but I'd rather not. > > >> > > >> >> 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 don't want > > >> >> to have to start decoding (2) by hand when debugging. > > >> > > > >> > Yep, that's a good point about readability. > > >> > > >> QMP sending all integers in decimal is inconvenient for some values, > > >> such as addresses. QMP sending all (large) integers in hexadecimal > > >> would be inconvenient for other values. > > >> > > >> Let's keep it simple & stupid. If you want sophistication, JSON is the > > >> wrong choice. > > >> > > >> > > >> 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 option 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 to > > > the application code taking the JSON parser/formatter out of the equation. > > > > Double-checking: do you propose to encode *all* numbers as strings, or > > just certain "problematic" numbers? > > > > If the latter, I guess your idea of "problematic" is "not representable > > exactly as double precision floating-point". > > We have a few options > > 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 > > 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). > > 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.