Re: [Qemu-devel] [PATCH v3 1/5] support nbd driver in blockdev-add

[Eric Blake]

On 09/14/2015 08:27 AM, Markus Armbruster wrote:
> Wen Congyang <address@hidden> writes:
> 
>> The NBD driver needs: filename, path or (host, port, exportname).
>> It checks which key exists and decides use unix or inet socket.
>> It doesn't recognize the key type, so we can't use union, and
>> can't reuse InetSocketAddress.
>>
>> Signed-off-by: Wen Congyang <address@hidden>
>> Signed-off-by: zhanghailiang <address@hidden>
>> Signed-off-by: Gonglei <address@hidden>
>> ---
>>  qapi/block-core.json | 42 ++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 40 insertions(+), 2 deletions(-)
>>

>>  ##
>> +# @BlockdevOptionsNBD
>> +#
>> +# Driver specific block device options for NBD
>> +#
>> +# @filename: #optional unix or inet path. The format is:
>> +#            unix: nbd+unix:///export?socket=path or
>> +#                  nbd:unix:path:exportname=export
>> +#            inet: nbd[+tcp]://host[:port]/export or
>> +#                  nbd:host[:port]:exportname=export

Yuck. You are passing structured data through a single 'str', when you
SHOULD be passing it through structured JSON.  Just because we have a
filename shorthand for convenience does NOT mean that we want to expose
that convenience in QMP.  Instead, we really want the breakdown of the
pieces (here, using abbreviated syntax of an inline base, since there
are pending qapi patches that will allow it):

{ 'enum': 'NBDTransport', 'data': ['unix', 'tcp', 'udp'] }
{ 'union': 'BlockdevOptionsNBD',
  'base': { 'transport': 'NBDTransport', 'export': 'str' },
  'discriminator': 'transport',
  'data': { 'unix': 'NBDUnix', 'tcp': 'NBDInet', 'udp': 'NBDInet' } }
{ 'struct': 'NBDUnix', 'data': { 'socket': 'str' } }
{ 'struct': 'NBDInet', 'data': { 'host': 'str', '*port': 'int',
     '*ipv4': 'bool', '*ipv6': 'bool' } }


> I'm afraid this doesn't address Eric's review of your v2.

Agreed; we still don't have the right interface.


> Eric further observed that support for the nbd+unix transport was
> missing, and suggested to have a union type combining the various
> transports.

And I just spelled out above what that should look like.

> 
> If we decide separate types for single port and port ranges aren't
> worthwhile, you can simply use SocketAddress where your v2 used
> InetSocketAddress.

I'm not sure if my 'NBDInet' above makes any more sense than reusing
'SocketAddress' (and if we do reuse 'SocketAddress', we have the further
question of whether to split out socket ranges as a separate type so
that SocketAddress becomes a single-port identity).

> 
> Eric, how strongly do you feel about separating the two?

I'm more worried about properly using a union type to represent unix vs.
tcp, and less worried about SocketAddress vs. range types vs creating a
new type (although in the long run, fixing ranges to be in a properly
named type rather than re-inventing the struct across multiple
transports is a good goal).  But you are quite correct that I do not
like the v3 proposal, because it encodes far too much information into a
single '*filename':'str', which is not the qapi way.

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

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