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

[Naveen N Rao <naveen@kernel.org>]

On Fri, Sep 11, 2026 at 11:55:07AM -0700, Sean Christopherson wrote:
> 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.

Agreed.

> 
> 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);
>   }

Unless you are planning to replace similar checks in __avic_vcpu_load() 
and __avic_vcpu_put(), I think it will be simpler to just check the id 
itself and avoid dealing with the table size:
	return vcpu->vcpu_id <= __avic_get_max_physical_id(kvm, NULL);

This helper can then also be used in avic_init_backing_page().

> 
> 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);
>   }
> 

Thanks,
Naveen