KVM: selftests: Assert in prepare_eptp() that nEPT is supported

Now that a VM isn't needed to check for nEPT support, assert that KVM
supports nEPT in prepare_eptp() instead of skipping the test, and push
the TEST_REQUIRE() check out to individual tests.  The require+assert are
somewhat redundant and will incur some amount of ongoing maintenance
burden, but placing the "require" logic in the test makes it easier to
find/understand a test's requirements and in this case, provides a very
strong hint that the test cares about nEPT.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20220927165209.930904-1-dmatlack@google.com
[sean: rebase on merged code, write changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/lib/x86_64/memstress.c

@@ -85,6 +85,7 @@ void memstress_setup_nested(struct kvm_vm *vm, int nr_vcpus, struct kvm_vcpu *vc
 	int vcpu_id;
 
 	TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_VMX));
+	TEST_REQUIRE(kvm_cpu_has_ept());
 
 	for (vcpu_id = 0; vcpu_id < nr_vcpus; vcpu_id++) {
 		vmx = vcpu_alloc_vmx(vm, &vmx_gva);

tools/testing/selftests/kvm/lib/x86_64/vmx.c

@@ -559,7 +559,7 @@ bool kvm_cpu_has_ept(void)
 void prepare_eptp(struct vmx_pages *vmx, struct kvm_vm *vm,
 		  uint32_t eptp_memslot)
 {
-	TEST_REQUIRE(kvm_cpu_has_ept());
+	TEST_ASSERT(kvm_cpu_has_ept(), "KVM doesn't support nested EPT");
 
 	vmx->eptp = (void *)vm_vaddr_alloc_page(vm);
 	vmx->eptp_hva = addr_gva2hva(vm, (uintptr_t)vmx->eptp);

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

@@ -78,6 +78,7 @@ int main(int argc, char *argv[])
 	bool done = false;
 
 	TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_VMX));
+	TEST_REQUIRE(kvm_cpu_has_ept());
 
 	/* Create VM */
 	vm = vm_create_with_one_vcpu(&vcpu, l1_guest_code);