Re: [PATCH 5/5] KVM: Convert the kvm->vcpus array to a xarray

[Marc Zyngier <maz@kernel.org>]

On Fri, 05 Nov 2021 20:21:36 +0000,
Sean Christopherson <seanjc@google.com> wrote:
> 
> On Fri, Nov 05, 2021, Marc Zyngier wrote:
> > At least on arm64 and x86, the vcpus array is pretty huge (512 entries),
> > and is mostly empty in most cases (running 512 vcpu VMs is not that
> > common). This mean that we end-up with a 4kB block of unused memory
> > in the middle of the kvm structure.
> 
> Heh, x86 is now up to 1024 entries.

Humph. I don't want to know whether people are actually using that in
practice. The only time I create VMs with 512 vcpus is to check
whether it still works...

>  
> > Instead of wasting away this memory, let's use an xarray instead,
> > which gives us almost the same flexibility as a normal array, but
> > with a reduced memory usage with smaller VMs.
> > 
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> > @@ -693,7 +694,7 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i)
> >  
> >  	/* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu.  */
> >  	smp_rmb();
> > -	return kvm->vcpus[i];
> > +	return xa_load(&kvm->vcpu_array, i);
> >  }
> 
> It'd be nice for this series to convert kvm_for_each_vcpu() to use
> xa_for_each() as well.  Maybe as a patch on top so that potential
> explosions from that are isolated from the initiali conversion?
> 
> Or maybe even use xa_for_each_range() to cap at online_vcpus?
> That's technically a functional change, but IMO it's easier to
> reason about iterating over a snapshot of vCPUs as opposed to being
> able to iterate over vCPUs as their being added.  In practice I
> doubt it matters.
> 
> #define kvm_for_each_vcpu(idx, vcpup, kvm) \
> 	xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, atomic_read(&kvm->online_vcpus))
>

I think that's already the behaviour of this iterator (we stop at the
first empty slot capped to online_vcpus. The only change in behaviour
is that vcpup currently holds a pointer to the last vcpu in no empty
slot has been encountered. xa_for_each{,_range}() would set the
pointer to NULL at all times.

I doubt anyone relies on that, but it is probably worth eyeballing
some of the use cases...

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.