Add a test to make sure that L2 toggling EFER.SVME works as intended.
This is a regression test for KVM mistakenly leaving nested and tearing
down nested state if L2 disables EFER.SVME, even though L1 is still
using nested.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
 tools/testing/selftests/kvm/Makefile.kvm      |  1 +
 .../kvm/x86/svm_nested_toggle_efer_svme.c     | 76 +++++++++++++++++++
 2 files changed, 77 insertions(+)
 create mode 100644 tools/testing/selftests/kvm/x86/svm_nested_toggle_efer_svme.c

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 58eee0474db6a..7bd2297b5494e 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -112,6 +112,7 @@ TEST_GEN_PROGS_x86 += x86/svm_vmcall_test
 TEST_GEN_PROGS_x86 += x86/svm_int_ctl_test
 TEST_GEN_PROGS_x86 += x86/svm_nested_shutdown_test
 TEST_GEN_PROGS_x86 += x86/svm_nested_soft_inject_test
+TEST_GEN_PROGS_x86 += x86/svm_nested_toggle_efer_svme
 TEST_GEN_PROGS_x86 += x86/tsc_scaling_sync
 TEST_GEN_PROGS_x86 += x86/sync_regs_test
 TEST_GEN_PROGS_x86 += x86/ucna_injection_test
diff --git a/tools/testing/selftests/kvm/x86/svm_nested_toggle_efer_svme.c b/tools/testing/selftests/kvm/x86/svm_nested_toggle_efer_svme.c
new file mode 100644
index 0000000000000..5267fbdbf01cc
--- /dev/null
+++ b/tools/testing/selftests/kvm/x86/svm_nested_toggle_efer_svme.c
@@ -0,0 +1,76 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2026, Google LLC.
+ */
+#include "kvm_util.h"
+#include "vmx.h"
+#include "svm_util.h"
+#include "kselftest.h"
+
+
+#define L2_GUEST_STACK_SIZE 64
+
+static void l2_guest_code(void)
+{
+	unsigned long efer = rdmsr(MSR_EFER);
+
+	/* generic_svm_setup() initializes EFER_SVME set for L2 */
+	GUEST_ASSERT(efer & EFER_SVME);
+
+	wrmsr(MSR_EFER, efer & ~EFER_SVME);
+	GUEST_ASSERT(rdmsr(MSR_EFER) == (efer & ~EFER_SVME));
+
+	wrmsr(MSR_EFER, efer);
+	GUEST_ASSERT(rdmsr(MSR_EFER) == efer);
+
+	vmmcall();
+}
+
+static void l1_guest_code(struct svm_test_data *svm)
+{
+	unsigned long l2_guest_stack[L2_GUEST_STACK_SIZE];
+
+	generic_svm_setup(svm, l2_guest_code,
+			  &l2_guest_stack[L2_GUEST_STACK_SIZE]);
+
+	run_guest(svm->vmcb, svm->vmcb_gpa);
+	GUEST_ASSERT(svm->vmcb->control.exit_code == SVM_EXIT_VMMCALL);
+
+	GUEST_DONE();
+}
+
+int main(int argc, char *argv[])
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	vm_vaddr_t nested_gva = 0;
+
+	TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_SVM));
+
+	vm = vm_create_with_one_vcpu(&vcpu, l1_guest_code);
+
+	vcpu_alloc_svm(vm, &nested_gva);
+	vcpu_args_set(vcpu, 1, nested_gva);
+
+	for (;;) {
+		struct ucall uc;
+
+		vcpu_run(vcpu);
+		TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_IO);
+
+		switch (get_ucall(vcpu, &uc)) {
+		case UCALL_ABORT:
+			REPORT_GUEST_ASSERT(uc);
+		case UCALL_SYNC:
+			break;
+		case UCALL_DONE:
+			goto done;
+		default:
+			TEST_FAIL("Unknown ucall %lu", uc.cmd);
+		}
+	}
+
+done:
+	kvm_vm_free(vm);
+	return 0;
+}
-- 
2.53.0.rc1.225.gd81095ad13-goog