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

Due to a likely mismerge of patches, KVM ended up with a superfluous
commit to "enable" AVIC's fast path for x2AVIC mode.  Even worse, the
superfluous commit has several bugs and creates a nasty local shadow
variable.

Rather than fix the bugs piece-by-piece[*] to achieve the same end
result, revert the patch wholesale.

Opportunistically add a comment documenting the x2AVIC dependencies.

This reverts commit 8c9e639d.

[*] https://lore.kernel.org/all/YxEP7ZBRIuFWhnYJ@google.com

Fixes: 8c9e639d ("KVM: SVM: Use target APIC ID to complete x2AVIC IRQs when possible")
Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230106011306.85230-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/avic.c

@@ -378,7 +378,17 @@ static int avic_kick_target_vcpus_fast(struct kvm *kvm, struct kvm_lapic *source
 
 		logid_index = cluster + __ffs(bitmap);
 
-		if (!apic_x2apic_mode(source)) {
+		if (apic_x2apic_mode(source)) {
+			/*
+			 * For x2APIC, the logical APIC ID is a read-only value
+			 * that is derived from the x2APIC ID, thus the x2APIC
+			 * ID can be found by reversing the calculation (done
+			 * above).  Note, bits 31:20 of the x2APIC ID are not
+			 * propagated to the logical ID, but KVM limits the
+			 * x2APIC ID limited to KVM_MAX_VCPU_IDS.
+			 */
+			l1_physical_id = logid_index;
+		} else {
 			u32 *avic_logical_id_table =
 				page_address(kvm_svm->avic_logical_id_table_page);
 
@@ -393,23 +403,6 @@ 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;
 		}
 	}