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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 18/06/2009 10:36, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>> 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?
> 
> 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)?

Or alternatively I would be happy to keep the full domctl functionality, but
then we have to use rcu for freeing the old vcpu array, and we have to
convince ourselves that arbitrarily increasing max_vcpus is safe (I'm not
sure what problems you foresee there?).

 -- Keir



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