Re: [PATCH v3 2/2] KVM: SVM: Limit AVIC physical max index based on configured max_vcpu_ids

["Gupta, Pankaj" <pankaj.gupta@xxxxxxx>]

On 2/20/2025 8:38 AM, Naveen N Rao (AMD) wrote:
> KVM allows VMMs to specify the maximum possible APIC ID for a virtual
> machine through KVM_CAP_MAX_VCPU_ID capability so as to limit data
> structures related to APIC/x2APIC. Utilize the same to set the AVIC
> physical max index in the VMCB, similar to VMX. This helps hardware
> limit the number of entries to be scanned in the physical APIC ID table
> speeding up IPI broadcasts for virtual machines with smaller number of
> vcpus.
>
> The minimum allocation required for the Physical APIC ID table is one 4k
> page supporting up to 512 entries. With AVIC support for 4096 vcpus
> though, it is sufficient to only allocate memory to accommodate the
> AVIC physical max index that will be programmed into the VMCB. Limit
> memory allocated for the Physical APIC ID table accordingly.
>
> Signed-off-by: Naveen N Rao (AMD) <naveen@xxxxxxxxxx>
Looks good to me. Liked the optimal solution.

Reviewed-by: Pankaj Gupta <pankaj.gupta@xxxxxxx>


> ---
>   arch/x86/kvm/svm/avic.c | 53 ++++++++++++++++++++++++++++++-----------
>   arch/x86/kvm/svm/svm.c  |  6 +++++
>   arch/x86/kvm/svm/svm.h  |  1 +
>   3 files changed, 46 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 1fb322d2ac18..dac4a6648919 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -85,6 +85,17 @@ struct amd_svm_iommu_ir {
>   	void *data;		/* Storing pointer to struct amd_ir_data */
>   };
>   
> +static inline u32 avic_get_max_physical_id(struct kvm *kvm, bool is_x2apic)
> +{
> +	u32 avic_max_physical_id = is_x2apic ? x2avic_max_physical_id : AVIC_MAX_PHYSICAL_ID;
> +
> +	/*
> +	 * Assume vcpu_id is the same as APIC ID. Per KVM_CAP_MAX_VCPU_ID, max_vcpu_ids
> +	 * represents the max APIC ID for this vm, rather than the max vcpus.
> +	 */
> +	return min(kvm->arch.max_vcpu_ids - 1, avic_max_physical_id);
> +}
> +
>   static void avic_activate_vmcb(struct vcpu_svm *svm)
>   {
>   	struct vmcb *vmcb = svm->vmcb01.ptr;
> @@ -103,7 +114,7 @@ static void avic_activate_vmcb(struct vcpu_svm *svm)
>   	 */
>   	if (x2avic_enabled && apic_x2apic_mode(svm->vcpu.arch.apic)) {
>   		vmcb->control.int_ctl |= X2APIC_MODE_MASK;
> -		vmcb->control.avic_physical_id |= x2avic_max_physical_id;
> +		vmcb->control.avic_physical_id |= avic_get_max_physical_id(svm->vcpu.kvm, true);
>   		/* Disabling MSR intercept for x2APIC registers */
>   		svm_set_x2apic_msr_interception(svm, false);
>   	} else {
> @@ -114,7 +125,7 @@ static void avic_activate_vmcb(struct vcpu_svm *svm)
>   		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, &svm->vcpu);
>   
>   		/* For xAVIC and hybrid-xAVIC modes */
> -		vmcb->control.avic_physical_id |= AVIC_MAX_PHYSICAL_ID;
> +		vmcb->control.avic_physical_id |= avic_get_max_physical_id(svm->vcpu.kvm, false);
>   		/* Enabling MSR intercept for x2APIC registers */
>   		svm_set_x2apic_msr_interception(svm, true);
>   	}
> @@ -174,6 +185,12 @@ int avic_ga_log_notifier(u32 ga_tag)
>   	return 0;
>   }
>   
> +static inline int avic_get_physical_id_table_order(struct kvm *kvm)
> +{
> +	/* Limit to the maximum physical ID supported in x2avic mode */
> +	return get_order((avic_get_max_physical_id(kvm, true) + 1) * sizeof(u64));
> +}
> +
>   void avic_vm_destroy(struct kvm *kvm)
>   {
>   	unsigned long flags;
> @@ -186,7 +203,7 @@ void avic_vm_destroy(struct kvm *kvm)
>   		__free_page(kvm_svm->avic_logical_id_table_page);
>   	if (kvm_svm->avic_physical_id_table_page)
>   		__free_pages(kvm_svm->avic_physical_id_table_page,
> -			     get_order(sizeof(u64) * (x2avic_max_physical_id + 1)));
> +			     avic_get_physical_id_table_order(kvm));
>   
>   	spin_lock_irqsave(&svm_vm_data_hash_lock, flags);
>   	hash_del(&kvm_svm->hnode);
> @@ -199,22 +216,12 @@ int avic_vm_init(struct kvm *kvm)
>   	int err = -ENOMEM;
>   	struct kvm_svm *kvm_svm = to_kvm_svm(kvm);
>   	struct kvm_svm *k2;
> -	struct page *p_page;
>   	struct page *l_page;
> -	u32 vm_id, entries;
> +	u32 vm_id;
>   
>   	if (!enable_apicv)
>   		return 0;
>   
> -	/* Allocating physical APIC ID table */
> -	entries = x2avic_max_physical_id + 1;
> -	p_page = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO,
> -			     get_order(sizeof(u64) * entries));
> -	if (!p_page)
> -		goto free_avic;
> -
> -	kvm_svm->avic_physical_id_table_page = p_page;
> -
>   	/* Allocating logical APIC ID table (4KB) */
>   	l_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
>   	if (!l_page)
> @@ -265,6 +272,24 @@ void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb)
>   		avic_deactivate_vmcb(svm);
>   }
>   
> +int avic_alloc_physical_id_table(struct kvm *kvm)
> +{
> +	struct kvm_svm *kvm_svm = to_kvm_svm(kvm);
> +	struct page *p_page;
> +
> +	if (kvm_svm->avic_physical_id_table_page || !enable_apicv || !irqchip_in_kernel(kvm))
> +		return 0;
> +
> +	p_page = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO,
> +			     avic_get_physical_id_table_order(kvm));
> +	if (!p_page)
> +		return -ENOMEM;
> +
> +	kvm_svm->avic_physical_id_table_page = p_page;
> +
> +	return 0;
> +}
> +
>   static u64 *avic_get_physical_id_entry(struct kvm_vcpu *vcpu,
>   				       unsigned int index)
>   {
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index b8aa0f36850f..3cb23298cdc3 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1423,6 +1423,11 @@ void svm_switch_vmcb(struct vcpu_svm *svm, struct kvm_vmcb_info *target_vmcb)
>   	svm->vmcb = target_vmcb->ptr;
>   }
>   
> +static int svm_vcpu_precreate(struct kvm *kvm)
> +{
> +	return avic_alloc_physical_id_table(kvm);
> +}
> +
>   static int svm_vcpu_create(struct kvm_vcpu *vcpu)
>   {
>   	struct vcpu_svm *svm;
> @@ -5007,6 +5012,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>   	.emergency_disable_virtualization_cpu = svm_emergency_disable_virtualization_cpu,
>   	.has_emulated_msr = svm_has_emulated_msr,
>   
> +	.vcpu_precreate = svm_vcpu_precreate,
>   	.vcpu_create = svm_vcpu_create,
>   	.vcpu_free = svm_vcpu_free,
>   	.vcpu_reset = svm_vcpu_reset,
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 5b159f017055..b4670afe0034 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -694,6 +694,7 @@ bool avic_hardware_setup(void);
>   int avic_ga_log_notifier(u32 ga_tag);
>   void avic_vm_destroy(struct kvm *kvm);
>   int avic_vm_init(struct kvm *kvm);
> +int avic_alloc_physical_id_table(struct kvm *kvm);
>   void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb);
>   int avic_incomplete_ipi_interception(struct kvm_vcpu *vcpu);
>   int avic_unaccelerated_access_interception(struct kvm_vcpu *vcpu);