Re: [Xen-devel] [PATCH v4 8/8] ioreq-server: bring the PCI hotplug controller implementation into Xen

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-04-08 at 09:25 +0100, Paul Durrant wrote:

> > Also, if you are obscuring those regions now how does it continue to
> > work?
> > 
> 
> We only obscure the old regions if we create the in-xen hotplug
> controller. This doesn't happen for migrated-in guests.

I presume it does happen for migrated in guests from new Xen with this
support?

> > Hrm, so I didn't see anything on the restore side which handles the
> > registration or not of the thing which would lead to EOPNOTSUPP vs
> > success on a guest started on an older Xen. How does all that actually
> > hang together?
> > 
> 
> If the guest was started on an older xen then
> HVM_PARAM_NR_IOREQ_SERVER_PAGES would not have been set, so when
> xc_domain_restore runs it would find no corresponding save record.
> Thus that param would not be set on restore and hence the controller
> would not be created. I could add another hvm op to make hotplug
> controller creation explicit if you like, but it seemed like rather a
> lot of extra code.

I think what wasn't obvious was that the use of the ioreq server
interfaces also implicitly causes the hotplug controller to be enabled
within Xen instead of elsewhere. I can see the chain of events which
leads to this now that it has been pointed out, and with that I can see
where the commit message implies it, but I think it could do with being
made explicit.

Ian.


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