KVM: selftests: Skip ICR.BUSY test in xapic_state_test if x2APIC is enabled

Don't test the ICR BUSY bit when x2APIC is enabled as AMD and Intel have
different behavior (AMD #GPs, Intel ignores), and the fact that the CPU
performs the reserved bit checks when IPI virtualization is enabled makes
it impossible for KVM to precisely emulate one or the other.

Link: https://lore.kernel.org/r/20240719235107.3023592-8-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/x86_64/xapic_state_test.c

@@ -70,12 +70,10 @@ static void ____test_icr(struct xapic_vcpu *x, uint64_t val)
 	vcpu_ioctl(vcpu, KVM_GET_LAPIC, &xapic);
 	icr = (u64)(*((u32 *)&xapic.regs[APIC_ICR])) |
 	      (u64)(*((u32 *)&xapic.regs[APIC_ICR2])) << 32;
-	if (!x->is_x2apic) {
+	if (!x->is_x2apic)
 		val &= (-1u | (0xffull << (32 + 24)));
+
 	TEST_ASSERT_EQ(icr, val & ~APIC_ICR_BUSY);
-	} else {
-		TEST_ASSERT_EQ(icr & ~APIC_ICR_BUSY, val & ~APIC_ICR_BUSY);
-	}
 }
 
 #define X2APIC_RSVED_BITS_MASK  (GENMASK_ULL(31,20) | \
@@ -91,7 +89,15 @@ static void __test_icr(struct xapic_vcpu *x, uint64_t val)
 		 */
 		val &= ~X2APIC_RSVED_BITS_MASK;
 	}
+
+	/*
+	 * The BUSY bit is reserved on both AMD and Intel, but only AMD treats
+	 * it is as _must_ be zero.  Intel simply ignores the bit.  Don't test
+	 * the BUSY bit for x2APIC, as there is no single correct behavior.
+	 */
+	if (!x->is_x2apic)
 		____test_icr(x, val | APIC_ICR_BUSY);
+
 	____test_icr(x, val & ~(u64)APIC_ICR_BUSY);
 }