KVM: selftests: Query module param to detect FEP in MSR filtering test

Add a helper to detect KVM support for forced emulation by querying the
module param, and use the helper to detect support for the MSR filtering
test instead of throwing a noodle/NOP at KVM to see if it sticks.

Cc: Aaron Lewis <aaronlewis@google.com>
Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20240109230250.424295-25-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/include/x86_64/processor.h

@@ -1222,6 +1222,11 @@ static inline bool kvm_is_pmu_enabled(void)
 	return get_kvm_param_bool("enable_pmu");
 }
 
+static inline bool kvm_is_forced_emulation_enabled(void)
+{
+	return !!get_kvm_param_integer("force_emulation_prefix");
+}
+
 uint64_t *__vm_get_page_table_entry(struct kvm_vm *vm, uint64_t vaddr,
 				    int *level);
 uint64_t *vm_get_page_table_entry(struct kvm_vm *vm, uint64_t vaddr);

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

@@ -14,8 +14,7 @@
 
 /* Forced emulation prefix, used to invoke the emulator unconditionally. */
 #define KVM_FEP "ud2; .byte 'k', 'v', 'm';"
-#define KVM_FEP_LENGTH 5
-static int fep_available = 1;
+static bool fep_available;
 
 #define MSR_NON_EXISTENT 0x474f4f00
 
@@ -260,13 +259,6 @@ static void guest_code_filter_allow(void)
 	GUEST_ASSERT(data == 2);
 	GUEST_ASSERT(guest_exception_count == 0);
 
-	/*
-	 * Test to see if the instruction emulator is available (ie: the module
-	 * parameter 'kvm.force_emulation_prefix=1' is set).  This instruction
-	 * will #UD if it isn't available.
-	 */
-	__asm__ __volatile__(KVM_FEP "nop");
-
 	if (fep_available) {
 		/* Let userspace know we aren't done. */
 		GUEST_SYNC(0);
@@ -388,12 +380,6 @@ static void guest_fep_gp_handler(struct ex_regs *regs)
 			   &em_wrmsr_start, &em_wrmsr_end);
 }
 
-static void guest_ud_handler(struct ex_regs *regs)
-{
-	fep_available = 0;
-	regs->rip += KVM_FEP_LENGTH;
-}
-
 static void check_for_guest_assert(struct kvm_vcpu *vcpu)
 {
 	struct ucall uc;
@@ -531,9 +517,11 @@ static void test_msr_filter_allow(void)
 {
 	struct kvm_vcpu *vcpu;
 	struct kvm_vm *vm;
+	uint64_t cmd;
 	int rc;
 
 	vm = vm_create_with_one_vcpu(&vcpu, guest_code_filter_allow);
+	sync_global_to_guest(vm, fep_available);
 
 	rc = kvm_check_cap(KVM_CAP_X86_USER_SPACE_MSR);
 	TEST_ASSERT(rc, "KVM_CAP_X86_USER_SPACE_MSR is available");
@@ -561,11 +549,11 @@ static void test_msr_filter_allow(void)
 	run_guest_then_process_wrmsr(vcpu, MSR_NON_EXISTENT);
 	run_guest_then_process_rdmsr(vcpu, MSR_NON_EXISTENT);
 
-	vm_install_exception_handler(vm, UD_VECTOR, guest_ud_handler);
 	vcpu_run(vcpu);
-	vm_install_exception_handler(vm, UD_VECTOR, NULL);
+	cmd = process_ucall(vcpu);
 
-	if (process_ucall(vcpu) != UCALL_DONE) {
+	if (fep_available) {
+		TEST_ASSERT_EQ(cmd, UCALL_SYNC);
 		vm_install_exception_handler(vm, GP_VECTOR, guest_fep_gp_handler);
 
 		/* Process emulated rdmsr and wrmsr instructions. */
@@ -583,6 +571,7 @@ static void test_msr_filter_allow(void)
 		/* Confirm the guest completed without issues. */
 		run_guest_then_process_ucall_done(vcpu);
 	} else {
+		TEST_ASSERT_EQ(cmd, UCALL_DONE);
 		printf("To run the instruction emulated tests set the module parameter 'kvm.force_emulation_prefix=1'\n");
 	}
 
@@ -804,6 +793,8 @@ static void test_user_exit_msr_flags(void)
 
 int main(int argc, char *argv[])
 {
+	fep_available = kvm_is_forced_emulation_enabled();
+
 	test_msr_filter_allow();
 
 	test_msr_filter_deny();