Re: [Xen-devel] [RFC PATCH 6/6] xc_version: add vm_event interface version

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 1/8/19 6:47 PM, Jan Beulich wrote:
> > > > On 08.01.19 at 17:37, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> > On 1/8/19 6:27 PM, Jan Beulich wrote:
> > > > > > On 19.12.18 at 19:52, <ppircalabu@xxxxxxxxxxxxxxx> wrote:
> > > > Signed-off-by: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>
> > > An empty description is not helpful. The immediate question is: Why?
> > > We don't do this for other interface versions. I'm unconvinced a
> > > special purpose piece of information like this one belongs into the
> > > rather generic version hypercall.
> > For an introspection application meant to be deployed on several Xen
> > versions without recompiling, it is important to be able to decide at
> > runtime what size and layout the vm_event struct has.
> >
> > Currently this can somewhat be done by associating the current version
> > with the vm_event version, but that is not ideal for obvious reasons.
> > Reading the vm_event version from an actual vm_event is also out of the
> > question, because in order to be able to receive the first vm_event we
> > have to set the ring buffer up, and that requires knowledge of the size
> > of the vm_event. So a run-time mechanism for querying the vm_event
> > version is needed.
> >
> > We just thought that this was the most flexible place to add it.
> How about a new XEN_DOMCTL_VM_EVENT_GET_VERSION?
That would work as well, we just thought this was the least intrusive 
and most extensible way to do it (other queries could be added similarly 
in the future, without needing a new DOMCTL / libxc toolstack 
modifications).

Thanks,
Razvan

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