Re: [Qemu-devel] ivshmem property size should be a size, not a string

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 11/20/2015 09:23 AM, Marc-André Lureau wrote:
>> Hi
>> 
>> ----- Original Message -----
>>> Everybody's favourite device model has "size" property.  It's declared
>>> as *string*
>>>
>>>     DEFINE_PROP_STRING("size", IVShmemState, sizearg),
>>>
>>>
>>> * In QMP, the size must be given as JSON string instead of JSON number,
>>>   and size suffixes are accepted.  Example: "size": "512k" instead of
>>>   "size": 524288.
>>>
>>>   Right now, this violation of QMP rules is tolerable (barely), because
>>>   device_add breaks some of these rules already.  However, one goal of
>>>   the current work on QAPI is to support a QMP command to plug devices
>>>   that doesn't break QMP rules, and then this violation will stand out.
>>>
>>>   Therefore, I want it fixed now, before ivshmem gets used in anger.
>
> Was ivshmem even usable in 2.4?  I'd argue that if we are going to
> change it, changing it for 2.5 is the ideal time.

Technically, we already have a compatibility break:

$ qemu-system-x86_64 -nodefaults -S -display none -chardev 
socket,path=/work/armbru/images/ivshmem-socket,id=ivshmem-chr -device 
ivshmem,size=4,chardev=ivshmem-chr,shm=foo

In 2.3.0, this ignores shm with warning "do not specify both 'chardev'
and 'shm' with ivshmem".

In current upstream, it's rejected.

I suspect we'd find more if we cared to look.

Of course, the only thing that *really* matters is *actual* usage in
anger: something of value that breaks.

Hash ivshmem been used in anger?  If yes, how?

>>>   A straight fix of size isn't fully backwards compatible: suffixes no
>>>   longer work in QMP, and you need to use an 'M' suffix to get Mebibytes
>>>   on command line and HMP.
>> 
>> I don't know the rules to break properties in qemu, but I would
>> prefer to avoid it if possible.
>
> It's possible to use an alternate to accept both a string and an
> integer.  But in general, QMP _wants_ to use byte-count integers without
> suffix (the convenience of '512k' is only for the command line and HMP),
> so letting QMP expose a string of "512k" instead of an integer 524288
> feels wrong.

Indeed.

>>>   If that's unacceptable, we'll have to provide a new, fixed property,
>>>   and deprecate size.
>> 
>> Sounds a better alternative to me.
>
> I'd rather fix the interface rather than catering to ugliness,
> particularly since 2.5 already fixes so much else that was ugly about
> ivshmem.
>
> Markus, did you have a patch to propose?

Working on one.