KVM: selftests: Update x86's KVM PV test to match KVM's disabling exits behavior

Rework x86's KVM PV features test to align with KVM's new, fixed behavior
of not allowing userspace to disable HLT-exiting after vCPUs have been
created. Rework the core testcase to disable HLT-exiting before creating
a vCPU, and opportunistically modify keep the paired VM+vCPU creation to
verify that KVM rejects KVM_CAP_X86_DISABLE_EXITS as expected.

Reviewed-by: Maxim Levitsky <[e-mail]>
Reviewed-by: Binbin Wu <[e-mail]>
Reviewed-by: Xiaoyao Li <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

tools/testing/selftests/kvm/x86/kvm_pv_test.c

···
 	struct kvm_vm *vm;
 	struct kvm_cpuid_entry2 *ent;
 	u32 kvm_sig_old;
+	int r;
 
 	if (!(kvm_check_cap(KVM_CAP_X86_DISABLE_EXITS) & KVM_X86_DISABLE_EXITS_HLT))
 		return;
···
 	TEST_ASSERT(vcpu_cpuid_has(vcpu, X86_FEATURE_KVM_PV_UNHALT),
 		    "Enabling X86_FEATURE_KVM_PV_UNHALT had no effect");
 
-	/* Make sure KVM clears vcpu->arch.kvm_cpuid */
+	/* Verify KVM disallows disabling exits after vCPU creation. */
+	r = __vm_enable_cap(vm, KVM_CAP_X86_DISABLE_EXITS, KVM_X86_DISABLE_EXITS_HLT);
+	TEST_ASSERT(r && errno == EINVAL,
+		    "Disabling exits after vCPU creation didn't fail as expected");
+
+	kvm_vm_free(vm);
+
+	/* Verify that KVM clear PV_UNHALT from guest CPUID. */
+	vm = vm_create(1);
+	vm_enable_cap(vm, KVM_CAP_X86_DISABLE_EXITS, KVM_X86_DISABLE_EXITS_HLT);
+
+	vcpu = vm_vcpu_add(vm, 0, NULL);
+	TEST_ASSERT(!vcpu_cpuid_has(vcpu, X86_FEATURE_KVM_PV_UNHALT),
+		    "vCPU created with PV_UNHALT set by default");
+
+	vcpu_set_cpuid_feature(vcpu, X86_FEATURE_KVM_PV_UNHALT);
+	TEST_ASSERT(!vcpu_cpuid_has(vcpu, X86_FEATURE_KVM_PV_UNHALT),
+		    "PV_UNHALT set in guest CPUID when HLT-exiting is disabled");
+
+	/*
+	 * Clobber the KVM PV signature and verify KVM does NOT clear PV_UNHALT
+	 * when KVM PV is not present, and DOES clear PV_UNHALT when switching
+	 * back to the correct signature..
+	 */
 	ent = vcpu_get_cpuid_entry(vcpu, KVM_CPUID_SIGNATURE);
 	kvm_sig_old = ent->ebx;
 	ent->ebx = 0xdeadbeef;
 	vcpu_set_cpuid(vcpu);
 
-	vm_enable_cap(vm, KVM_CAP_X86_DISABLE_EXITS, KVM_X86_DISABLE_EXITS_HLT);
+	vcpu_set_cpuid_feature(vcpu, X86_FEATURE_KVM_PV_UNHALT);
+	TEST_ASSERT(vcpu_cpuid_has(vcpu, X86_FEATURE_KVM_PV_UNHALT),
+		    "PV_UNHALT cleared when using bogus KVM PV signature");
+
 	ent = vcpu_get_cpuid_entry(vcpu, KVM_CPUID_SIGNATURE);
 	ent->ebx = kvm_sig_old;
 	vcpu_set_cpuid(vcpu);
 
 	TEST_ASSERT(!vcpu_cpuid_has(vcpu, X86_FEATURE_KVM_PV_UNHALT),
-		    "KVM_FEATURE_PV_UNHALT is set with KVM_CAP_X86_DISABLE_EXITS");
+		    "PV_UNHALT set in guest CPUID when HLT-exiting is disabled");
 
 	/* FIXME: actually test KVM_FEATURE_PV_UNHALT feature */