Re: [Qemu-devel] nbd: Possible regression in 2.9 RCs

[Eric Blake]

On 04/04/2017 07:16 AM, Kevin Wolf wrote:

> 
> Now the big question is how to implement this. Just not requesting the
> write permission initially if runstate_check(RUN_STATE_INMIGRATE) is
> easy. But we need to find a place to actually request it later, after
> the mirror has completed, and before the VM is running.
> 
> My first thought was that we could add a VMChangeStateHandler and just
> request the permission in there. However, requesting the permission can
> fail (e.g. because the NBD server hasn't been shut down) and we can't
> let a state transition fail from a VMChangeStateHandler.
> 
> Maybe the next best thing we could do is to have a BlockDevOps callback
> for invalidate_cache/inactivate and get the permissions there. This one
> could return an error, which would be passed up the stack and eventually
> cause qmp_cont() to fail (i.e. it would refuse to resume the VM). This
> sounds workable to me.

Having 'cont' refuse when write permissions can't be grabbed after all
sounds reasonable (assuming, of course, that the common case is that the
invalidate_cache/inactivate call that triggers the callback will
normally succeed, because it means the source is now done and the
destination _should_ be able to get write access at that point).

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

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