Re: [Qemu-devel] [PATCH v8 03/11] netfilter: add netfilter_{add|del} commands

[Eric Blake <eblake@redhat.com>]

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On 08/26/2015 09:17 AM, Markus Armbruster wrote:
> Only reviewing QAPI/QMP and HMP interface parts for now.
> 
> I apologize for not having reviewed this series earlier.  v8 is awfully
> late for the kind of review comments I have.

And I've also been behind the curve, intending to review this since it
touches API, but not getting there yet.


>> +##
>> +{ 'command': 'netfilter_add',
>> +  'data': {
>> +    'type': 'str',
>> +    'id':   'str',
>> +    'netdev': 'str',
>> +    '*chain': 'str',
>> +    '*props': '**'}, 'gen': false }
> 
> I figure you're merely following netdev_add precedence here (can't fault
> you for that), but netdev_add cheats, and we shouldn't add more cheats.
> 
> First, 'gen': false is best avoided.  It suppresses the generated
> marshaller, and that lets you cheat.  There are cases where we can't do
> without, but I don't think this is one.
> 
> When we suppress the generated marshaller, 'data' is at best a
> declaration of intent; the code can do something else entirely.  For
> instance, netdev_add declares
> 
>     { 'command': 'netdev_add',
>       'data': {'type': 'str', 'id': 'str', '*props': '**'},
>       'gen': false }
> 
> but the '*props' part is a bald-faced lie: it doesn't take a 'props'
> argument.  See
> http://lists.gnu.org/archive/html/qemu-devel/2015-08/msg00460.html
> and maybe even slides 37-38 of
> https://events.linuxfoundation.org/sites/events/files/slides/armbru-qemu-introspection.pdf
> 
> I didn't check your code, but I suspect yours is a lie, too.
> 
> I intend to clean up netdev_add, hopefully soon.
> 
> You should use a proper QAPI data type here.  I guess the flat union I
> sketched in my reply to PATCH 2 would do nicely, except we don't support
> commands with union type data, yet.  I expect to add support to clean up
> netdev_del.

In fact, I've already proposed adding such support:

http://thread.gmane.org/gmane.comp.emulators.qemu/356265/focus=356291

> 
> If you don't want to wait for that (understandable!), then I suggest you
> keep 'NetFilter' a struct type for now, use it as command data here, and
> we convert it to a flat union later.  Must be done before the release,
> to avoid backward incompatibility.
> 
> Then this becomes something like:
> 
>     { 'command': 'netfilter-add', 'data': 'NetFilter' }

or use NetFilter as a union, but have the command be:

{ 'command': 'netfilter-add',
  'data': { 'data': 'NetFilter' } }

where you have to pass an extra layer of nesting at the QMP layer.

> 
> If you need the command to take arguments you don't want to put into
> NetFilter for some reason, I can help you achieve that cleanly.

In fact, having the 'NetFilter' union be a single argument of a larger
struct makes that larger struct the nice place to stick any additional
arguments that don't need to be part of the union.

> 
>> +
>> +##
>> +# @netfilter_del:
>> +#
>> +# Remove a netfilter.
>> +#
>> +# @id: the name of the netfilter to remove
>> +#
>> +# Returns: Nothing on success
>> +#          If @id is not a valid netfilter, DeviceNotFound
>> +#
>> +# Since: 2.5
>> +##
>> +{ 'command': 'netfilter_del', 'data': {'id': 'str'} }

Please name new QMP commands with '-' not '_'; this should be
'netfilter-del'.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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