KVM: Convert the kvm->vcpus array to a xarray

At least on arm64 and x86, the vcpus array is pretty huge (up to
1024 entries on x86) and is mostly empty in the majority of the cases
(running 1k 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.

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>
Message-Id: <20211116160403.4074052-6-maz@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0a504c7988dc..594f90307b20 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -458,7 +458,7 @@ void kvm_destroy_vcpus(struct kvm *kvm)
 
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		kvm_vcpu_destroy(vcpu);
-		kvm->vcpus[i] = NULL;
+		xa_erase(&kvm->vcpu_array, i);
 	}
 
 	atomic_set(&kvm->online_vcpus, 0);
@@ -1063,6 +1063,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
 	mutex_init(&kvm->slots_arch_lock);
 	spin_lock_init(&kvm->mn_invalidate_lock);
 	rcuwait_init(&kvm->mn_memslots_update_rcuwait);
+	xa_init(&kvm->vcpu_array);
 
 	INIT_LIST_HEAD(&kvm->devices);
 
@@ -3598,7 +3599,10 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
 	}
 
 	vcpu->vcpu_idx = atomic_read(&kvm->online_vcpus);
-	BUG_ON(kvm->vcpus[vcpu->vcpu_idx]);
+	r = xa_insert(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, GFP_KERNEL_ACCOUNT);
+	BUG_ON(r == -EBUSY);
+	if (r)
+		goto unlock_vcpu_destroy;
 
 	/* Fill the stats id string for the vcpu */
 	snprintf(vcpu->stats_id, sizeof(vcpu->stats_id), "kvm-%d/vcpu-%d",
@@ -3608,15 +3612,14 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, u32 id)
 	kvm_get_kvm(kvm);
 	r = create_vcpu_fd(vcpu);
 	if (r < 0) {
+		xa_erase(&kvm->vcpu_array, vcpu->vcpu_idx);
 		kvm_put_kvm_no_destroy(kvm);
 		goto unlock_vcpu_destroy;
 	}
 
-	kvm->vcpus[vcpu->vcpu_idx] = vcpu;
-
 	/*
-	 * Pairs with smp_rmb() in kvm_get_vcpu.  Write kvm->vcpus
-	 * before kvm->online_vcpu's incremented value.
+	 * Pairs with smp_rmb() in kvm_get_vcpu.  Store the vcpu
+	 * pointer before kvm->online_vcpu's incremented value.
 	 */
 	smp_wmb();
 	atomic_inc(&kvm->online_vcpus);