Daniel P. Berrangé <berrange@xxxxxxxxxx> writes:
> On Mon, May 13, 2019 at 01:29:34PM +0100, Dr. David Alan Gilbert wrote:
>> * Daniel P. Berrangé (berrange@xxxxxxxxxx) wrote:
>> > On Wed, May 08, 2019 at 02:44:07PM +0200, Markus Armbruster wrote:
>> > > Daniel P. Berrangé <berrange@xxxxxxxxxx> 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?
>
> 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.
Yes, QAPI type 'int' is int64_t. All of QAPI's integer types map to
exact-width C integer types.
>>
>> > 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.
If we had many places where we could forget to handle the
interoperability capability, I'd shoot down the idea of having it :)
Fortunately, we don't: the qobject visitors take care of it, and they
come with unit tests.
I listed the few exceptions above under "Let's talk implementation for a
bit."
* query-qmp-schema
No input.
Output contains no numbers. Should it acquire numbers, we'll probably
want to get rid of the hack, so the output goes through the visitor.
* netdev_add
Input gets stringified anyway (misfeature).
No output.
* device_add
Input gets stringified anyway (misfeature).
No output.
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