Re: [PATCH v4 6/7] KVM: SVM: Add support for AMD IOMMU Guest APIC Physical Processor Interrupt (GAPPI)

[Vasant Hegde]

On 8/21/2026 11:26 AM, Sairaj Kodilkar wrote:
> With AVIC guest-mode interrupt remapping, device interrupts are posted into
> the guest vAPIC backing page by the IOMMU. When the vCPU is not running
> (IRTE[IsRun] = 0), KVM must still be notified to schedule it. The legacy
> path uses the GA log.
> 
> GAPPI (Guest APIC Physical Processor Interrupt) is an alternative to the
> GA log mechanism provided by the AMD IOMMU. With GAPPI enabled, the IOMMU
> still updates the vAPIC backing page IRR, but the host wakeup notification
> is delivered as a physical APIC interrupt to IRTE[Destination], using
> IRTE[GATag][7:0] as the vector (POSTED_INTR_WAKEUP_VECTOR).
> 
> SVM follows the Intel posted-interrupt wakeup model. Each pCPU maintains
> a list of blocked vCPUs that may be woken by a GAPPI delivery to that CPU.
> When a vCPU blocks while waiting for an interrupt, SVM enqueues it on the
> wakeup list of the pCPU on which it was previously running and passes that
> same pCPU's physical APIC ID to the IOMMU to program IRTE[Destination].
> The rationale is that the vCPU is likely to run again on the same pCPU,
> which is common when vCPUs are pinned; targeting GAPPI notifications there
> reduces unnecessary VMEXITs from GAPPI deliveries on other CPUs. SVM
> registers the GAPPI handler via kvm_set_posted_intr_wakeup_handler(). On
> delivery, it walks the local vCPU list and wakes vCPUs with a pending IRR.
> 
> All GAPPI logic is gated on amd_iommu_gappi. Without it, KVM and the IOMMU
> falls back to the legacy GA log mechanism for vCPU wakeup.
> 
> Signed-off-by: Sairaj Kodilkar <sarunkod@xxxxxxx>
> ---
>  arch/x86/kvm/svm/avic.c | 72 ++++++++++++++++++++++++++++++++++++-----
>  arch/x86/kvm/svm/svm.c  |  3 ++
>  arch/x86/kvm/svm/svm.h  |  5 +++
>  3 files changed, 72 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index dd497530d365..18ac24ef40e1 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -874,6 +874,8 @@ int avic_init_vcpu(struct vcpu_svm *svm)
>  	INIT_LIST_HEAD(&svm->ir_list);
>  	raw_spin_lock_init(&svm->ir_list_lock);
>  
> +	svm->gappi_cpu = -1;

Can we initialize it to CPU0?
> +
>  	if (!enable_apicv || !irqchip_in_kernel(vcpu->kvm))
>  		return 0;
>  
> @@ -886,6 +888,20 @@ int avic_init_vcpu(struct vcpu_svm *svm)
>  	return ret;
>  }
>  
> +void avic_destroy_vcpu(struct vcpu_svm *svm)
> +{
> +	if (amd_iommu_gappi && svm->gappi_cpu != -1) {

Redundant amd_iommu_gappi check?

> +		unsigned long flags;
> +
> +		local_irq_save(flags);
> +
> +		kvm_pi_disable_wakeup_handler(&svm->vcpu, svm->gappi_cpu);
> +		svm->gappi_cpu = -1;

Redundant assignement?

-Vasant