Re: [PATCH] KVM: SVM: Clear AVIC Physical ID table entry if vCPU creation fails

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 03, 2026, Naveen N Rao (AMD) wrote:
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 3b037e385523..bc2c699380b3 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -885,6 +885,16 @@ int avic_init_vcpu(struct vcpu_svm *svm)
>  	return ret;
>  }
>  
> +void avic_vcpu_free(struct kvm_vcpu *vcpu)
> +{
> +	u32 max_id = x2avic_enabled ? x2avic_max_physical_id : AVIC_MAX_PHYSICAL_ID;

I don't love the duplicate (triplicate?) code, and looking at the usage in
avic_init_backing_page() with fresh eyes sketched me out.  It's "fine", because
KVM will reject vCPU creation if vcpu_id >= kvm->arch.max_vcpu_ids, i.e. checking
only the architectural max won't exceed this max:

	return min(kvm->arch.max_vcpu_ids - 1, arch_max);

But it's hard to see that, and I can't think of any reason why being paranoid
during vCPU creation/destruction would be a bad thing.

Assuming it actually works (haven't tested yet), I'll send a v2 with a prep patch
to add:

  static bool avic_is_addressable_vcpu(struct kvm_vcpu *vcpu)
  {
	return (vcpu->vcpu_id * sizeof(u64)) <
	       PAGE_SIZE << avic_get_physical_id_table_order(vcpu->kvm);
  }

and then this code becomes:

  void avic_vcpu_free(struct kvm_vcpu *vcpu)
  {
	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);

	if (kvm_svm->avic_physical_id_table && avic_is_addressable_vcpu(vcpu))
		WRITE_ONCE(kvm_svm->avic_physical_id_table[vcpu->vcpu_id], 0);
  }

> +	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
> +	u32 id = vcpu->vcpu_id;
> +
> +	if (kvm_svm->avic_physical_id_table && id <= max_id)
> +		WRITE_ONCE(kvm_svm->avic_physical_id_table[id], 0);
> +}