Re: [Xen-devel] [PATCH 3/3] x86_64: allow more vCPU-s per guest

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Keir Fraser <keir.fraser@xxxxxxxxxxxxx> 18.06.09 11:36 >>>
>On 18/06/2009 10:27, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:
>
>>> Applied. Is the vcpu[] array re-allocation in XEN_DOMCTL_max_vcpus an
>>> example of over-optimistic-concurrency-control? It can't really be 100% safe
>>> without extra locking on all users of that array (not good), or using rcu
>>> (better), can it. The wmb() is a nice try. ;-)
>> 
>> Agreed. Originally I intended to add at least a comment, but after realizing
>> that the path currently is only reached during domain creation (and I believe
>> there are issues elsewhere if one would really allow increasing the # of
>> vCPU-s in a domain on the fly), I decided to leave it as is (and the wmb()
>> can be considered sort of a comment to that effect).
>
>Then I think it would be better to make that domctl really singleshot (i.e.,
>fail if d->max_vcpus is already non-zero), with a comment explaining why it
>is implemented this way. Buggily implementing an unused case can't be good.
>Can you generate a patch for this, please?

Sure.

>Is it safe do you think to go from d->max_vcpus=0 to d->max_vcpus!=0 on a
>live domain? (i.e, the specific case we do use in your general observation
>that increasing d->max_vcpus may be dangerous)?

Yes, this is safe imo.

Jan



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