Re: [virtio-comment] [PATCH v1 1/8] admin: Add theory of operation for device migration

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

On Fri, Nov 17, 2023 at 10:20:45AM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Friday, November 17, 2023 3:38 PM
> > 
> > On Wed, Nov 15, 2023 at 05:39:43PM +0000, Parav Pandit wrote:
> > > > >
> > > > > Additionally, if hypervisor has put the trap on virtio config, and
> > > > > because the memory device already has the interface for virtio
> > > > > config,
> > > > >
> > > > > Hypervisor can directly write/read from the virtual config to the
> > > > > member's
> > > > config space, without going through the device context, right?
> > > >
> > > > If it can do it or it can choose to not. I don't see how it is
> > > > related to the discussion here.
> > > >
> > > It is. I don’t see a point of hypervisor not using the native interface provided
> > by the member device.
> > 
> > So for example, it seems reasonable to a member supporting both existing pci
> > register interface for compatibility and the future DMA based one for scale. In
> > such a case, it seems possible that DMA will expose more features than pci. And
> > then a hypervisor might decide to use that in preference to pci registers.
> 
> We don’t find it right to involve owner device for mediating at
> current scale

In this model, device will be its own owner. Should not be a problem.

> and to not break TDISP efforts in upcoming time by such
> design.

Look you either stop mentioning TDISP as motivation or actually
try to address it. Safe migration with TDISP is really hard.
For example, your current patches are clearly broken for TDISP:
owner can control queue state at any time making device modify
memory in any way it wants.

> And for future scale, having new SIOV interface makes more sense which has its own direct interface to device.
> 
> I finally captured all past discussions in form of a FAQ at [1].
> 
> [1] https://docs.google.com/document/d/1Iyn-l3Nm0yls3pZaul4lZiVj8x1s73Ed6rOsmn6LfXc/edit?usp=sharing

Yea skimmed that, "Cons: None". Are you 100% sure? Anyway, discussion
will take place on the mailing list please. 

-- 
MST


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