[PATCH 4/5] KVM: selftests: Explicitly verify KVM doesn't patch hypercall if quirk==off

[Sean Christopherson <seanjc@google.com>]

Explicitly verify that KVM doesn't patch in the native hypercall if the
FIX_HYPERCALL_INSN quirk is disabled.  The test currently verifies that
a #UD occurred, but doesn't actually verify that no patching occurred.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 .../selftests/kvm/x86_64/fix_hypercall_test.c | 35 ++++++++++++++-----
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c b/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
index dde97be3e719..5925da3b3648 100644
--- a/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
+++ b/tools/testing/selftests/kvm/x86_64/fix_hypercall_test.c
@@ -21,8 +21,8 @@ static bool ud_expected;
 
 static void guest_ud_handler(struct ex_regs *regs)
 {
-	GUEST_ASSERT(ud_expected);
-	GUEST_DONE();
+	regs->rax = -EFAULT;
+	regs->rip += HYPERCALL_INSN_SIZE;
 }
 
 extern unsigned char svm_hypercall_insn[HYPERCALL_INSN_SIZE];
@@ -57,17 +57,18 @@ static void guest_main(void)
 {
 	unsigned char *native_hypercall_insn, *hypercall_insn;
 	uint8_t apic_id;
+	uint64_t ret;
 
 	apic_id = GET_APIC_ID_FIELD(xapic_read_reg(APIC_ID));
 
 	if (is_intel_cpu()) {
 		native_hypercall_insn = vmx_hypercall_insn;
 		hypercall_insn = svm_hypercall_insn;
-		svm_do_sched_yield(apic_id);
+		ret = svm_do_sched_yield(apic_id);
 	} else if (is_amd_cpu()) {
 		native_hypercall_insn = svm_hypercall_insn;
 		hypercall_insn = vmx_hypercall_insn;
-		vmx_do_sched_yield(apic_id);
+		ret = vmx_do_sched_yield(apic_id);
 	} else {
 		GUEST_ASSERT(0);
 		/* unreachable */
@@ -75,12 +76,28 @@ static void guest_main(void)
 	}
 
 	/*
-	 * The hypercall didn't #UD (guest_ud_handler() signals "done" if a #UD
-	 * occurs).  Verify that a #UD is NOT expected and that KVM patched in
-	 * the native hypercall.
+	 * If the quirk is disabled, verify that guest_ud_handler() "returned"
+	 * -EFAULT and that KVM did NOT patch the hypercall.  If the quirk is
+	 * enabled, verify that the hypercall succeeded and that KVM patched in
+	 * the "right" hypercall.
 	 */
-	GUEST_ASSERT(!ud_expected);
-	GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn, HYPERCALL_INSN_SIZE));
+	if (ud_expected) {
+		GUEST_ASSERT(ret == (uint64_t)-EFAULT);
+
+		/*
+		 * Divergence should occur only on the last byte, as the VMCALL
+		 * (0F 01 C1) and VMMCALL (0F 01 D9) share the first two bytes.
+		 */
+		GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn,
+				     HYPERCALL_INSN_SIZE - 1));
+		GUEST_ASSERT(memcmp(native_hypercall_insn, hypercall_insn,
+				    HYPERCALL_INSN_SIZE));
+	} else {
+		GUEST_ASSERT(!ret);
+		GUEST_ASSERT(!memcmp(native_hypercall_insn, hypercall_insn,
+			     HYPERCALL_INSN_SIZE));
+	}
+
 	GUEST_DONE();
 }
 
-- 
2.37.2.789.g6183377224-goog