KVM: selftests: Test x2APIC ICR reserved bits

Actually test x2APIC ICR reserved bits instead of deliberately skipping
them.  The behavior that is observed when IPI virtualization is enabled is
the architecturally correct behavior, KVM is the one who was wrong, i.e.
KVM was missing reserved bit checks.

Fixes: 4b88b1a ("KVM: selftests: Enhance handling WRMSR ICR register in x2APIC mode")
Link: https://lore.kernel.org/r/20240719235107.3023592-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

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

@@ -31,6 +31,10 @@ static void xapic_guest_code(void)
 	}
 }
 
+#define X2APIC_RSVD_BITS_MASK  (GENMASK_ULL(31, 20) | \
+				GENMASK_ULL(17, 16) | \
+				GENMASK_ULL(13, 13))
+
 static void x2apic_guest_code(void)
 {
 	asm volatile("cli");
@@ -41,7 +45,10 @@ static void x2apic_guest_code(void)
 		uint64_t val = x2apic_read_reg(APIC_IRR) |
 			       x2apic_read_reg(APIC_IRR + 0x10) << 32;
 
-		x2apic_write_reg(APIC_ICR, val);
+		if (val & X2APIC_RSVD_BITS_MASK)
+			x2apic_write_reg_fault(APIC_ICR, val);
+		else
+			x2apic_write_reg(APIC_ICR, val);
 		GUEST_SYNC(val);
 	} while (1);
 }
@@ -72,24 +79,14 @@ static void ____test_icr(struct xapic_vcpu *x, uint64_t val)
 	      (u64)(*((u32 *)&xapic.regs[APIC_ICR2])) << 32;
 	if (!x->is_x2apic)
 		val &= (-1u | (0xffull << (32 + 24)));
+	else if (val & X2APIC_RSVD_BITS_MASK)
+		return;
 
 	TEST_ASSERT_EQ(icr, val & ~APIC_ICR_BUSY);
 }
 
-#define X2APIC_RSVED_BITS_MASK  (GENMASK_ULL(31,20) | \
-				 GENMASK_ULL(17,16) | \
-				 GENMASK_ULL(13,13))
-
 static void __test_icr(struct xapic_vcpu *x, uint64_t val)
 {
-	if (x->is_x2apic) {
-		/* Hardware writing vICR register requires reserved bits 31:20,
-		 * 17:16 and 13 kept as zero to avoid #GP exception. Data value
-		 * written to vICR should mask out those bits above.
-		 */
-		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