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

[Sean Christopherson <seanjc@google.com>]

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.
 
> 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))
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