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KVM: SVM: Limit AVIC physical max index based on configured max_vcpu_ids

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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.

Unlike VMX, SVM AVIC requires a single page to be allocated for the
Physical APIC ID table and the Logical APIC ID table, so retain the
existing approach of allocating those during VM init.

Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/adb07ccdb3394cd79cb372ba6bcc69a4e4d4ef54.1757009416.git.naveen@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Naveen N Rao 2025-09-05 00:03:01 +05:30 committed by Sean Christopherson
parent c53c632592
commit 574ef752d4
1 changed files with 5 additions and 2 deletions
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7
arch/x86/kvm/svm/avic.c
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@ -161,6 +161,7 @@ static void avic_set_x2apic_msr_interception(struct vcpu_svm *svm,
static void avic_activate_vmcb(struct vcpu_svm *svm)
{
struct vmcb *vmcb = svm->vmcb01.ptr;
struct kvm *kvm = svm->vcpu.kvm;
vmcb->control.int_ctl &= ~(AVIC_ENABLE_MASK | X2APIC_MODE_MASK);
vmcb->control.avic_physical_id &= ~AVIC_PHYSICAL_MAX_INDEX_MASK;
@ -176,7 +177,8 @@ 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 |= min(kvm->arch.max_vcpu_ids - 1,
X2AVIC_MAX_PHYSICAL_ID);
/* Disabling MSR intercept for x2APIC registers */
avic_set_x2apic_msr_interception(svm, false);
} else {
@ -187,7 +189,8 @@ 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 |= min(kvm->arch.max_vcpu_ids - 1,
AVIC_MAX_PHYSICAL_ID);
/* Enabling MSR intercept for x2APIC registers */
avic_set_x2apic_msr_interception(svm, true);
}