[PATCH v3 55/62] KVM: SVM: Use vcpu_idx, not vcpu_id, for GA log tag/metadata

Sean Christopherson
2025-06-11T22:48:31Z

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>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/avic.c | 37 ++++++++++++++++++++-----------------
1 file changed, 20 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index c55cbb0610b4..bb74705d6cfd 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -29,36 +29,39 @@
#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);
@@ -139,16 +142,16 @@ 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);
@@ -785,7 +788,7 @@ 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,
--
2.50.0.rc1.591.g9c95f17f64-goog