Re: [virtio-comment] Re: [PATCH v1 3/8] device-context: Define the device context fields for device migration

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 26, 2023 at 08:56:47AM +0800, Jason Wang wrote:
> > > We transfer data by DMA, the device writes DMA dirty pages information(bitmap)
> > > to host isolated memory region.
> > >
> >
> >
> > If you do that then I don't see any reason not to use admin
> > commands for that - either through a vq or a simpler
> > interface.
> 
> I think we need to agree that admin commands are the only interface
> for any future features before we can have an agreement here.

I don't think that needs to be the case. I do think that if
your goal is a separate channel from normal device operation
then this is what admin commands have been designed for.

> My understanding is that it is optional for the transport that
> requires administrative commands like provisioning etc. It is not
> necessarily the interface for new features.

Yes. And migration is IMO sufficiently "like provisioning".

> >
> >
> > >
> > >         Config space interfaces are fundamental for virtio-pci.
> > >
> > >
> > >     They are in fact fundamental to virtio. Multiple transports to
> > >     use config space are also fundamental.
> > >
> > > I agree. So I also agree to build admin vq live migration solution based on our
> > > basic facilities, as Jason ever proposed.
> >
> >
> > I'm not sure it's even a vq. I suggest a minimal interface to send
> > admin commands. Could be used by migration, as transport, and more.
> >
> 
> It's better if we can do that below the layer of admin commands. For
> example, we don't stick device status with any specific interface. We
> can keep doing things like this.
> 
> Thanks

Could go either way, but complex functionality like live migration
can benefit from a rich interface.

-- 
MST


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