Re: [PATCH 0/4] KVM: add KVM_CREATE_VM2 to allow dynamic kvm->vcpus array

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-04-18 14:29+0200, Cornelia Huck:
> On Tue, 18 Apr 2017 13:11:55 +0200
> David Hildenbrand <david@redhat.com> wrote:
>> On 13.04.2017 22:19, Radim Krčmář wrote:
>> > new KVM_MAX_CONFIGURABLE_VCPUS, probably directly to INT_MAX/KVM_VCPU_ID, so we
>> > don't have to worry about it for a while.
>> > 
>> > PPC should be interested in this as they set KVM_MAX_VCPUS to NR_CPUS
>> > and probably waste few pages for every guest this way.
>> 
>> As we just store pointers, this should be a maximum of 4 pages for ppc
>> (4k pages). Is this really worth yet another VM creation ioctl? Is there
>> not a nicer way to handle this internally?
>> 
>> An alternative might be to simply realloc the array when it reaches a
>> certain size (on VCPU creation, maybe protecting the pointer via rcu).
>> But not sure if something like that could work.
> 
> I like that idea better, if it does work (I think it should be doable).
> If we just double the array size every time we run out of space, we
> should be able to do this with few reallocations. That has also the
> advantage of being transparent to user space (other than increased
> number of vcpus).

Yes, relocating would work with protection against use-after-free and
RCU fits well.  (Readers don't have any lock we could piggyback on.)

I didn't go for it because of C: the kvm_for_each_vcpu macro would be
less robust if it included the locking around its body -- nested fors are
susceptible to return/goto errors inside the loop body + we'd need to
obfuscate several existing users of that pattern.  And open-coding the
protection everywhere is polluting the code too, IMO.

Lock-less list would solve those problems, but we are accessing the
VCPUs by index, which makes it suboptimal in other direction ... using
the list for kvm_for_each_vcpu and adding RCU protected array for
kvm_get_vcpu and kvm_get_vcpu_by_id looks like over-engineering as we
wouldn't save memory, performance, nor lines of code by doing that.

I didn't see a way to untangle kvm->vcpu that would allow a nice
runtime-dynamic variant.

We currently don't need to pass more information at VM creation time
either, so I was also thinking of hijacking the parameter to
KVM_CREATE_VM for factor-of-2 VCPU count (20 bits would last a while),
but that is already a new interface and new IOCTL to do a superset of
another one seemed much better.

I agree that the idea is questionable.   I'll redo the series and bump
KVM_MAX_VCPUS unless you think that the dynamic could be done nicely.

(The memory saving is a miniscule fraction of a VM size and if we do big
 increments in KVM_MAX_VCPUS, then the motivation is gone.)

Thanks.