Re: [Qemu-devel] [PATCH v2 0/5] Add ignore-external migration capability

[Daniel P . Berrangé]

On Mon, Feb 11, 2019 at 04:03:57PM +0000, Dr. David Alan Gilbert wrote:
> * Yury Kotov (address@hidden) wrote:
> > Hi,
> > 
> > The series adds a migration capability, which allows to skip shared RAM 
> > blocks
> > during the migration. It's useful for fast local migration. E.g. to update 
> > QEMU
> > for the running guests.
> > 
> > Usage example:
> > 1. Start source VM:
> >    qemu-system-x86 \
> >      -m 4G \
> >      -object 
> > memory-backend-file,id=mem0,size=4G,share=on,mem-path=/dev/shm/mem0 \
> >      -numa node,memdev=mem0 \
> >      -qmp unix:/tmp/qemu-qmp-1.sock,server,nowait \
> > 
> > 2. Start target VM:
> >    qemu-system-x86 \
> >      -m 4G \
> >      -object 
> > memory-backend-file,id=mem0,size=4G,share=on,mem-path=/dev/shm/mem0 \
> >      -numa node,memdev=mem0 \
> >      -qmp unix:/tmp/qemu-qmp-2.sock,server,nowait \
> >      -incoming defer
> > 
> > 3. Enable ignore-external capability on both VMs:
> >    { "execute": "migrate-set-capabilities" , "arguments":
> >      { "capabilities": [ { "capability": "x-ignore-external", "state": true 
> > } ] } }
> > 
> > 4. Start migration.
> > 
> > Another use case I keep in mind is to migrate to file. Usage is very 
> > similar.
> 
> Hi,
>   I've cc'd in Eric, Lai and Peter, all who were asking for something
> similar last year;  can you all confirm this patch does what you need or
> can work with what you needed to do?

x-ignore-external is a global setting so affects all memory-backend-file
instances. The obvious question is where there is any reasonable/conceivable
scenario in which QEMU would have multiple "-object memory-backend-file"
intsances and it be neccessary/desirable to migrate some, but skip migrate
of others ?

Could there ever be a device backend which is using a memory region
to communicate with an external process, where you would need to have
a new instance for the migration target QEMU, and explicitly not reuse
the source QEMU's memory-backend-file storage  ?

Regards,
Daniel
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