Re: [Xen-devel] [PATCH 4/6] domctl: set XEN_DOMCTL_createdomain 'rover' if valid domid is specified

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

Paul Durrant writes ("[PATCH 4/6] domctl: set XEN_DOMCTL_createdomain 'rover' 
if valid domid is specified"):
> The value of 'rover' is the value at which Xen will start searching for an
> unused domid if none is specified. Currently it is only updated when a
> domid is automatically chosen, rather than specified by the caller, which
> makes it very hard to describe Xen's rationale in choosing domids in an
> environment where some domain creations have specified domids and some
> don't.
> This patch always updates 'rover' after a successful creation, even in the
> case that domid is specified by the caller. This ensures that, if Xen
> automatically chooses a domid for a subsequent domain creation it will
> always be the next available value after the domid of the most recently
> created domain.

I'm not yet convinced this behaviour is better, but I'm open to
persuasion.

The existing allocation system falls down in any case if the domid
gets near to wrapping round.  If it doesn't, then without this patch
it is possible to have two ranges of domids: automatically allocated
ones, and statically allocated high ones.

With this patch, statically allocating a domid resets rover and causes
the remaining bits of static space to be polluted.

What am I missing ?  What are the use cases here ?

Thanks,
Ian.

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