Re: [Qemu-devel] [PATCH 0/5] vhost-scsi: Add support for host virtualized target

[Paolo Bonzini]

Il 28/01/2013 14:36, Michael S. Tsirkin ha scritto:
> On Mon, Jan 28, 2013 at 02:29:23PM +0100, Paolo Bonzini wrote:
>> Il 28/01/2013 14:11, Michael S. Tsirkin ha scritto:
>>>> I asked for a standalone device because the configuration mechanism
>>>> (configfs vs. command-line) and the feature set are completely
>>>> different.  Unlike virtio-net, it's not possible to switch one to the
>>>> other at run time.
>>>
>>> Exactly the same applies to any other frontend option.
>>> For example if you have two qemu instances with
>>> different num_queues values you can not migrate one
>>> to the other.
>>> So in this sense it is not different from any other
>>> frontend option, right?
>>
>> Indeed, in this sense it is not.
>>
>> Actually in this case migrating one to the other could succeed, and make
>> all disks disappear on the destination (because of the different
>> configuration mechanism).  That however could be overcome with vhost=on
>> registering a migration blocker.
> 
> Or better add a subsection if vhost is set: vhost=on to vhost=on
> can migrate, right?

I think it's not yet supported by the kernel.  You have no guarantee
that I/O is quiescent at the time the VM starts on the destination.
You'd need a ioctl to do the equivalent of bdrv_drain_all().

Once you have that, a subsection would do the job, yes.

Paolo

>> I won't really block the patch with the vhost=on/off frontend option if
>> it is properly done (e.g. the QEMU SCSI bus should not be created for
>> vhost=on) and minimally invasive to the non-vhost code.
>>
>> Paolo