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 05:42:56AM +0000, Parav Pandit wrote:
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, November 1, 2023 11:01 AM
> > 
> > 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.
> > 
> Most control structs are well defined. So only its type field is added to migrating driver side.
> This is very low overhead field and handled in generic way for all device types and for all common types.

Weird, not what I see.  E.g. you seem to have a structure duplicating queue
fields. Each new field will have to be added there in addition to the
transport.

> > 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.
> I didn't follow the suggestion. Can you explain splitting patches and its relation to the structure?
> 
> > 
> > Another is to add MSIX table migration option for when MSIX table is passed
> > through to guest.
> Yes, this will be added in future when there is actual hypervisor for it.

You are tying the architecture to an extremely implementation specific
detail.

Hypervisors *already* have migrate the MSIX table. Just in a hypervisor
specific way. queue vector is an index into this table. So the index
is migrated through the device but the table itself has to be
trapped and emulated by hypervisor? Give me a break.

-- 
MST


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