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David,

On Thu, Feb 11, 2021 at 4:54 AM David Gibson
<kata-dev at gibson.dropbear.id.au> wrote:
>
> Hi, hoping somebody can clear up a couple of points of confusion for
> me.
>
> 1) The shimv2 protocol
>
> AIUI, the shimv2 protocol has basically the same semantics as the OCI
> runtime spec, but is over gRPC instead of a command line interface.
> Is that correct?  Is there somewhere to find the spec of the shimv2
> protocol?

Not exactly. There's a bunch of APIs that are the same, but it's not a
1:1 relationship.
Please, refer to the following document for more details:
https://github.com/containerd/containerd/issues/2426

>
>
> 2) CRI-O "runtime_type" option
>
> The CRI-O configuration has a "runtime_type" variable for each
> runtime, with options of "oci" or "vm".  What exactly the difference
> between those is kind of unclear: what docs I've found say that "vm"
> is more suitable for VM based runtimes, but I can't really see any
> reason that would make a difference for crio.  Looking at the code it
> looks like "vm" switches CRI-O to use the "shimv2" grpc based
> interface rather than command line.  It makes sense CRI-O would need
> to know that, but AFAICT it has nothing whatsoever to do with whether
> the runtime uses a VM.

My guess is that it inherited its "vm" name because the shimv2
proposal had in mind VM based runtimes.
As you mentioned, CRI-O has to know it's dealing with "shimv2", and
that's pretty much about it.

Best Regards,
-- 
Fabiano Fidêncio