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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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.


Thanks,
Razvan

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