Re: [Qemu-devel] [PATCH] qemu: json: Fix parsing of integers >= 0x8000000000000000

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/23/2011 10:19 AM, Richard W.M. Jones wrote:
> On Mon, May 23, 2011 at 10:07:21AM -0500, Anthony Liguori wrote:
>> On 05/23/2011 09:29 AM, Markus Armbruster wrote:
>>> Anthony Liguori<anthony@codemonkey.ws>   writes:
>>>
>>> JavaScript's implementation of JSON sets limits on the range of numbers,
>>> namely they need to fit into IEEE doubles.
>>>
>>> Our implementation sets different limits.  IIRC, it's something like
>>> "numbers with a fractional part or an exponent need to fit into IEEE
>>> doubles, anything else into int64_t."  Not exactly the acme of elegance,
>>> either.  But it works for us.
>>
>> In order to be compatible with JavaScript (which I think is
>> necessary to really satisfy the spec), we just need to make sure
>> that our integers are represented by at least 53-bits (to enable
>> signed integers) and critically, fall back to floating point
>> representation to ensure that we round instead of truncate.
>
> The problem is to be able to send 64 bit memory and disk offsets
> faithfully.  This doesn't just fail to solve the problem, it's
> actually going to make it a whole lot worse.
>
> I don't agree with you that whatever the JSON standard (RFC) doesn't
> specify must be filled in by reading Javascript/ECMA.

"  It is derived from the object
    literals of JavaScript, as defined in the ECMAScript Programming
    Language Standard, Third Edition [ECMA]."

> If this is so
> important, it's very odd that it doesn't mention this fallback in the
> RFC.  If you read the RFC alone then it's pretty clear (to me) that it
> leaves limits up to the application.

If it's left up to the application, doesn't that mean that we can't ever 
send 64-bit memory/disk faithfully?

Because a client would be allowed to represent integers as signed 32-bit 
numbers.

Fundamentally, we need to ask ourselves, do we want to support any JSON 
client or require JSON libraries explicitly written for QEMU?

What I suggested would let us work with any JSON client, but if clients 
loose precision after 53-bits, those clients would not work well with QEMU.

The alternative approach is to be conservative and only use 32-bit 
integers and represent everything in two numbers.

Regards,

Anthony Liguori

>
> Rich.
>