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 Wed, Nov 01, 2023 at 02:54:47AM +0000, Parav Pandit wrote:
> 
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Tuesday, October 31, 2023 3:44 PM
> > 
> > On Tue, Oct 31, 2023 at 05:42:29PM +0800, Zhu, Lingshan wrote:
> > > > Your answer is not relevant to this discussion at all.
> > > > Why?
> > > > Because we were discussing the schemes where registers are not used.
> > > > One example of that was IMS. It does not matter MSI or MSIX.
> > > > As explained in Intel's commit message, the key to focus for IMS is "queue
> > memory" not some hw register like MSI or MSI-X.
> > > you know the device always need to know a address and the data to send
> > > a MSI, right?
> > 
> > So if virtio is to use IMS then we'll need to add interfaces to program IMS, I
> > think. As part of that patch - it's reasonable to assume - we will also need to add
> > a way to retrieve IMS so it can be migrated.
> > 
> > However, what this example demonstrates is that the approach taken by this
> > proposal to migrate control path structures - namely, by defining a structure
> > used just for migration - means that we will need to come up with a migration
> > interface each time.
> > And that is unfortunate.
> >
> When the device supports a new feature it has supported new functionality.
> Hence the live migration side also got updated.
> However, the live migration driver does not have to understand what is inside the control path structures.
> It is just byte stream.
> Only if the hypervisor live migration drive involved in emulating, it will parse and that is fine as like other control structures.

The point is that any new field needs to be added in two places now and
that is not great at all.

We need a stronger compatiblity story here I think.

One way to show how it's designed to work would be to split
the patches. For example, add queue notify data and queue reset
separately.

Another is to add MSIX table migration option for when MSIX table
is passed through to guest.

-- 
MST