KVM: SVM: Use vcpu_idx, not vcpu_id, for GA log tag/metadata

Use a vCPU's index, not its ID, for the GA log tag/metadata that's used to
find and kick vCPUs when a device posted interrupt serves as a wake event.
Lookups on a vCPU index are O(fast) (not sure what xa_load() actually
provides), whereas a vCPU ID lookup is O(n) if a vCPU's ID doesn't match
its index.

Unlike the Physical APIC Table, which is accessed by hardware when
virtualizing IPIs, hardware doesn't consume the GA tag, i.e. KVM _must_
use APIC IDs to fill the Physical APIC Table, but KVM has free rein over
the format/meaning of the GA tag.

Tested-by: Sairaj Kodilkar <sarunkod@amd.com>
Link: https://lore.kernel.org/r/20250611224604.313496-57-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/avic.c

@@ -30,36 +30,39 @@
 #include "svm.h"
 
 /*
- * Encode the arbitrary VM ID and the vCPU's default APIC ID, i.e the vCPU ID,
- * into the GATag so that KVM can retrieve the correct vCPU from a GALog entry
- * if an interrupt can't be delivered, e.g. because the vCPU isn't running.
+ * Encode the arbitrary VM ID and the vCPU's _index_ into the GATag so that
+ * KVM can retrieve the correct vCPU from a GALog entry if an interrupt can't
+ * be delivered, e.g. because the vCPU isn't running.  Use the vCPU's index
+ * instead of its ID (a.k.a. its default APIC ID), as KVM is guaranteed a fast
+ * lookup on the index, where as vCPUs whose index doesn't match their ID need
+ * to walk the entire xarray of vCPUs in the worst case scenario.
  *
- * For the vCPU ID, use however many bits are currently allowed for the max
+ * For the vCPU index, use however many bits are currently allowed for the max
  * guest physical APIC ID (limited by the size of the physical ID table), and
  * use whatever bits remain to assign arbitrary AVIC IDs to VMs.  Note, the
  * size of the GATag is defined by hardware (32 bits), but is an opaque value
  * as far as hardware is concerned.
  */
-#define AVIC_VCPU_ID_MASK		AVIC_PHYSICAL_MAX_INDEX_MASK
+#define AVIC_VCPU_IDX_MASK		AVIC_PHYSICAL_MAX_INDEX_MASK
 
 #define AVIC_VM_ID_SHIFT		HWEIGHT32(AVIC_PHYSICAL_MAX_INDEX_MASK)
 #define AVIC_VM_ID_MASK			(GENMASK(31, AVIC_VM_ID_SHIFT) >> AVIC_VM_ID_SHIFT)
 
 #define AVIC_GATAG_TO_VMID(x)		((x >> AVIC_VM_ID_SHIFT) & AVIC_VM_ID_MASK)
-#define AVIC_GATAG_TO_VCPUID(x)		(x & AVIC_VCPU_ID_MASK)
+#define AVIC_GATAG_TO_VCPUIDX(x)	(x & AVIC_VCPU_IDX_MASK)
 
-#define __AVIC_GATAG(vm_id, vcpu_id)	((((vm_id) & AVIC_VM_ID_MASK) << AVIC_VM_ID_SHIFT) | \
-					 ((vcpu_id) & AVIC_VCPU_ID_MASK))
-#define AVIC_GATAG(vm_id, vcpu_id)					\
+#define __AVIC_GATAG(vm_id, vcpu_idx)	((((vm_id) & AVIC_VM_ID_MASK) << AVIC_VM_ID_SHIFT) | \
+					 ((vcpu_idx) & AVIC_VCPU_IDX_MASK))
+#define AVIC_GATAG(vm_id, vcpu_idx)					\
 ({									\
-	u32 ga_tag = __AVIC_GATAG(vm_id, vcpu_id);			\
+	u32 ga_tag = __AVIC_GATAG(vm_id, vcpu_idx);			\
 									\
-	WARN_ON_ONCE(AVIC_GATAG_TO_VCPUID(ga_tag) != (vcpu_id));	\
+	WARN_ON_ONCE(AVIC_GATAG_TO_VCPUIDX(ga_tag) != (vcpu_idx));	\
 	WARN_ON_ONCE(AVIC_GATAG_TO_VMID(ga_tag) != (vm_id));		\
 	ga_tag;								\
 })
 
-static_assert(__AVIC_GATAG(AVIC_VM_ID_MASK, AVIC_VCPU_ID_MASK) == -1u);
+static_assert(__AVIC_GATAG(AVIC_VM_ID_MASK, AVIC_VCPU_IDX_MASK) == -1u);
 
 static bool force_avic;
 module_param_unsafe(force_avic, bool, 0444);
@@ -140,16 +143,16 @@ int avic_ga_log_notifier(u32 ga_tag)
 	struct kvm_svm *kvm_svm;
 	struct kvm_vcpu *vcpu = NULL;
 	u32 vm_id = AVIC_GATAG_TO_VMID(ga_tag);
-	u32 vcpu_id = AVIC_GATAG_TO_VCPUID(ga_tag);
+	u32 vcpu_idx = AVIC_GATAG_TO_VCPUIDX(ga_tag);
 
-	pr_debug("SVM: %s: vm_id=%#x, vcpu_id=%#x\n", __func__, vm_id, vcpu_id);
-	trace_kvm_avic_ga_log(vm_id, vcpu_id);
+	pr_debug("SVM: %s: vm_id=%#x, vcpu_idx=%#x\n", __func__, vm_id, vcpu_idx);
+	trace_kvm_avic_ga_log(vm_id, vcpu_idx);
 
 	spin_lock_irqsave(&svm_vm_data_hash_lock, flags);
 	hash_for_each_possible(svm_vm_data_hash, kvm_svm, hnode, vm_id) {
 		if (kvm_svm->avic_vm_id != vm_id)
 			continue;
-		vcpu = kvm_get_vcpu_by_id(&kvm_svm->kvm, vcpu_id);
+		vcpu = kvm_get_vcpu(&kvm_svm->kvm, vcpu_idx);
 		break;
 	}
 	spin_unlock_irqrestore(&svm_vm_data_hash_lock, flags);
@@ -786,7 +789,7 @@ int avic_pi_update_irte(struct kvm_kernel_irqfd *irqfd, struct kvm *kvm,
 		 */
 		struct amd_iommu_pi_data pi_data = {
 			.ga_tag = AVIC_GATAG(to_kvm_svm(kvm)->avic_vm_id,
-					     vcpu->vcpu_id),
+					     vcpu->vcpu_idx),
 			.is_guest_mode = kvm_vcpu_apicv_active(vcpu),
 			.vapic_addr = avic_get_backing_page_address(to_svm(vcpu)),
 			.vector = vector,