Re: [Qemu-devel] export root node for write through NBD

[Vladimir Sementsov-Ogievskiy]

30.11.2017 17:33, Kevin Wolf wrote:
> Am 30.11.2017 um 15:21 hat Max Reitz geschrieben:
> > On 2017-11-30 08:47, Vladimir Sementsov-Ogievskiy wrote:
> > > Hi all.
> > >
> > > We need the following option: start vm in stopped mode (-S) and write
> > > it's disk before start through NBD.
> > > It should be absolutely safe, but unfortunately it is disallowed by root
> > > role of the disk.
> > > Is there any workaround or if not, what is a true way to implement this
> > > possibility?
> > >
> > > ----
> > > error message:
> > >      unable to execute QEMU command 'nbd-server-add': Conflicts with use
> > > by drive0 as 'root', which does not allow 'write' on #block100
> > One thing that comes to mind is adding the guest device only after you
> > are done with NBD.
> >
> > The other of course is to set share-rw=on for the device.
> We already allow this with incoming migration, in that case the
> BlockBackend for the guest device stays inactive and doesn't request
> permissions yet.
>
> I've been thinking for a while that it would be nice to unify the
> startup of "normal" VMs and incoming migration, so that images are
> always opened inactive first and then get activated.
>
> The interesting point for you could be that opening the image happens
> immediately when the process starts, but activation would only happen
> when the VM actually starts running (i.e. on 'cont').
>
> Kevin

Could it be as simple as:

diff --git a/block/block-backend.c b/block/block-backend.c
index 3064cb888f..b8cbb43f02 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -709,7 +709,9 @@ static int blk_do_attach_dev(BlockBackend *blk, void 
*dev)
     /* While migration is still incoming, we don't need to apply the
      * permissions of guest device BlockBackends. We might still have 
a block
      * job or NBD server writing to the image for storage migration. */
-    if (runstate_check(RUN_STATE_INMIGRATE)) {
+    if (runstate_check(RUN_STATE_INMIGRATE) ||
+        runstate_check(RUN_STATE_PRELAUNCH))
+    {
         blk->disable_perm = true;
     }


(at least it fixes my case)

--
Best regards,
Vladimir