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

[Jason Wang <jasowang@redhat.com>]

å 2023/10/26 14:22, Michael S. Tsirkin åé:
> 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".


To me, this looks more like a transport. Hypervisors can trap and 
forward it to admin virtqueue. This somehow duplicates with the idea of 
transport q over admin.


> > > >          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.


That's my understanding as well.

Thanks


>