Re: [Qemu-devel] [PATCH v4 11/11] nbd-server: Allow node name for nbd-server-add

[Max Reitz]

On 14.07.2016 23:36, Eric Blake wrote:
> On 07/14/2016 07:28 AM, Kevin Wolf wrote:
>> There is no reason why an NBD server couldn't be started for any node,
>> even if it's not on the top level. This converts nbd-server-add to
>> accept a node-name.
>>
>> Note that there is a semantic difference between using a BlockBackend
>> name and the node name of its root: In the former case, the NBD server
>> is closed on eject; in the latter case, the NBD server doesn't drop its
>> reference and keeps the image file open this way.
>>
>> Signed-off-by: Kevin Wolf <address@hidden>
>> ---
>>  blockdev-nbd.c  | 21 +++++++++------------
>>  qapi/block.json |  4 ++--
>>  2 files changed, 11 insertions(+), 14 deletions(-)
>>
>> diff --git a/blockdev-nbd.c b/blockdev-nbd.c
>> index c437d32..ca41cc6 100644
>> --- a/blockdev-nbd.c
>> +++ b/blockdev-nbd.c
>> @@ -145,7 +145,8 @@ void qmp_nbd_server_start(SocketAddress *addr,
>>  void qmp_nbd_server_add(const char *device, bool has_writable, bool 
>> writable,
>>                          Error **errp)
>>  {
>> -    BlockBackend *blk;
>> +    BlockDriverState *bs = NULL;
>> +    BlockBackend *on_eject_blk;
>>      NBDExport *exp;
>>  
>>      if (!nbd_server) {
>> @@ -158,26 +159,22 @@ void qmp_nbd_server_add(const char *device, bool 
>> has_writable, bool writable,
>>          return;
> 
> Do we want to do any sanity checking that writing should only be
> permitted on a root, and that when using a node name that is not a root
> that writable must be false so as not to negatively change the BDS out
> of under the feet of the other root?  Do op-blockers already cover that?

Well, one could argue that it's possible to create an NBD server on a
non-root node today anyway, since creating BBs is not restricted to root
nodes:

blockdev-add(id=foo, other arguments...)
blockdev-add(id=bar, backing=foo, other arguments...)

And then you can create an NBD server on bar. I agree that this is not
how it should be, though. However, I think that the fact that you need
to specify a BB name for now deters people from doing stuff like that.
If you can specify a node name, people will think it's completely fine
to do so.

Also note that only allowing NBD servers to be created on a root node
doesn't really help you:

blockdev-add(node-name=foo, ...)
nbd-server-add(device=foo)
blockdev-add(id=bar, backing=foo, ...)

So, yeah, I think we just need the new op-blockers for this, I don't
think the current op blockers cover this.

Max

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