KVM: SVM: Require logical ID to be power-of-2 for AVIC entry

Do not modify AVIC's logical ID table if the logical ID portion of the
LDR is not a power-of-2, i.e. if the LDR has multiple bits set.  Taking
only the first bit means that KVM will fail to match MDAs that intersect
with "higher" bits in the "ID"

The "ID" acts as a bitmap, but is referred to as an ID because there's an
implicit, unenforced "requirement" that software only set one bit.  This
edge case is arguably out-of-spec behavior, but KVM cleanly handles it
in all other cases, e.g. the optimized logical map (and AVIC!) is also
disabled in this scenario.

Refactor the code to consolidate the checks, and so that the code looks
more like avic_kick_target_vcpus_fast().

Fixes: 18f40c5 ("svm: Add VMEXIT handlers for AVIC")
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20230106011306.85230-28-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: sean-jc

arch/x86/kvm/svm/avic.c

@@ -513,26 +513,26 @@ unsigned long avic_vcpu_get_apicv_inhibit_reasons(struct kvm_vcpu *vcpu)
 static u32 *avic_get_logical_id_entry(struct kvm_vcpu *vcpu, u32 ldr, bool flat)
 {
 	struct kvm_svm *kvm_svm = to_kvm_svm(vcpu->kvm);
-	int index;
 	u32 *logical_apic_id_table;
-	int dlid = GET_APIC_LOGICAL_ID(ldr);
+	u32 cluster, index;
 
-	if (!dlid)
-		return NULL;
-
-	if (flat) { /* flat */
-		index = ffs(dlid) - 1;
-		if (index > 7)
-			return NULL;
-	} else { /* cluster */
-		int cluster = (dlid & 0xf0) >> 4;
-		int apic = ffs(dlid & 0x0f) - 1;
+	ldr = GET_APIC_LOGICAL_ID(ldr);
 
-		if ((apic < 0) || (apic > 7) ||
-		    (cluster >= 0xf))
+	if (flat) {
+		cluster = 0;
+	} else {
+		cluster = (ldr >> 4);
+		if (cluster >= 0xf)
 			return NULL;
-		index = (cluster << 2) + apic;
+		ldr &= 0xf;
 	}
+	if (!ldr || !is_power_of_2(ldr))
+		return NULL;
+
+	index = __ffs(ldr);
+	if (WARN_ON_ONCE(index > 7))
+		return NULL;
+	index += (cluster << 2);
 
 	logical_apic_id_table = (u32 *) page_address(kvm_svm->avic_logical_id_table_page);