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

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

On 10/31/2023 6:14 PM, Michael S. Tsirkin wrote:
> 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.
>
> Compare to the trap and emulate approach for config space and we don't
> need a new interface, we just make each field R/W.
> So I feel this is something to think about, and address.
> Ideas?
I think this is transport-specific. For example, PCI has a MSI tables, 
driver can access MSI info through the
MSI entries in the table, That is R/W for migration.

For SIOV, we have invented "transport vq", tvq support set/get MSI 
configurations,
I will continue this tvq task once live migration solution merged.
>