KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible

For x2AVIC, the index from incomplete IPI #vmexit info is invalid
for logical cluster mode. Only ICRH/ICRL values can be used
to determine the IPI destination APIC ID.

Since QEMU defines guest physical APIC ID to be the same as
vCPU ID, it can be used to quickly identify the target vCPU to deliver IPI,
and avoid the overhead from searching through all vCPUs to match the target
vCPU.
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Message-Id: <20220519102709.24125-16-suravee.suthikulpanit@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/avic.c

@@ -390,9 +390,7 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
 
 		logid_index = cluster + __ffs(bitmap);
 
-		if (apic_x2apic_mode(source)) {
-			l1_physical_id = logid_index;
-		} else {
+		if (!apic_x2apic_mode(source)) {
 			u32 *avic_logical_id_table =
 				page_address(kvm_svm->avic_logical_id_table_page);
 
@@ -407,6 +405,23 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
 
 			l1_physical_id = logid_entry &
 					 AVIC_LOGICAL_ID_ENTRY_GUEST_PHYSICAL_ID_MASK;
+		} else {
+			/*
+			 * For x2APIC logical mode, cannot leverage the index.
+			 * Instead, calculate physical ID from logical ID in ICRH.
+			 */
+			int cluster = (icrh & 0xffff0000) >> 16;
+			int apic = ffs(icrh & 0xffff) - 1;
+
+			/*
+			 * If the x2APIC logical ID sub-field (i.e. icrh[15:0])
+			 * contains anything but a single bit, we cannot use the
+			 * fast path, because it is limited to a single vCPU.
+			 */
+			if (apic < 0 || icrh != (1 << apic))
+				return -EINVAL;
+
+			l1_physical_id = (cluster << 4) + apic;
 		}
 	}