[PATCH v2 03/13] KVM: selftests: Add a test for LBR save/restore (ft. nested)

Yosry Ahmed
2025-11-10T22:29:48Z

Add a selftest exercising save/restore with usage of LBRs in both L1 and
L2, and making sure all LBRs remain intact.

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
---
tools/testing/selftests/kvm/Makefile.kvm | 1 +
.../selftests/kvm/include/x86/processor.h | 5 +
.../selftests/kvm/x86/svm_lbr_nested_state.c | 155 ++++++++++++++++++
3 files changed, 161 insertions(+)
create mode 100644 tools/testing/selftests/kvm/x86/svm_lbr_nested_state.c

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 148d427ff24be..9a19554ffd3c1 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -105,6 +105,7 @@ 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_lbr_nested_state
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/include/x86/processor.h b/tools/testing/selftests/kvm/include/x86/processor.h
index 51cd84b9ca664..aee4b83c47b19 100644
--- a/tools/testing/selftests/kvm/include/x86/processor.h
+++ b/tools/testing/selftests/kvm/include/x86/processor.h
@@ -1367,6 +1367,11 @@ static inline bool kvm_is_ignore_msrs(void)
return get_kvm_param_bool("ignore_msrs");
}

+static inline bool kvm_is_lbrv_enabled(void)
+{
+ return !!get_kvm_amd_param_integer("lbrv");
+}
+
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);
diff --git a/tools/testing/selftests/kvm/x86/svm_lbr_nested_state.c b/tools/testing/selftests/kvm/x86/svm_lbr_nested_state.c
new file mode 100644
index 0000000000000..a343279546fd8
--- /dev/null
+++ b/tools/testing/selftests/kvm/x86/svm_lbr_nested_state.c
@@ -0,0 +1,155 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * svm_lbr_nested_state
+ *
+ * Test that LBRs are maintained correctly in both L1 and L2 during
+ * save/restore.
+ *
+ * Copyright (C) 2025, Google, Inc.
+ */
+
+#include "test_util.h"
+#include "kvm_util.h"
+#include "processor.h"
+#include "svm_util.h"
+
+
+#define L2_GUEST_STACK_SIZE 64
+
+#define DO_BRANCH() asm volatile("jmp 1f\n 1: nop")
+
+struct lbr_branch {
+ u64 from, to;
+};
+
+volatile struct lbr_branch l2_branch;
+
+#define RECORD_BRANCH(b, s) \
+({ \
+ wrmsr(MSR_IA32_DEBUGCTLMSR, DEBUGCTLMSR_LBR); \
+ DO_BRANCH(); \
+ (b)->from = rdmsr(MSR_IA32_LASTBRANCHFROMIP); \
+ (b)->to = rdmsr(MSR_IA32_LASTBRANCHTOIP); \
+ /* Disabe LBR right after to avoid overriding the IPs */ \
+ wrmsr(MSR_IA32_DEBUGCTLMSR, 0); \
+ \
+ GUEST_ASSERT_NE((b)->from, 0); \
+ GUEST_ASSERT_NE((b)->to, 0); \
+ GUEST_PRINTF("%s: (0x%lx, 0x%lx)\n", (s), (b)->from, (b)->to); \
+}) \
+
+#define CHECK_BRANCH_MSRS(b) \
+({ \
+ GUEST_ASSERT_EQ((b)->from, rdmsr(MSR_IA32_LASTBRANCHFROMIP)); \
+ GUEST_ASSERT_EQ((b)->to, rdmsr(MSR_IA32_LASTBRANCHTOIP)); \
+})
+
+#define CHECK_BRANCH_VMCB(b, vmcb) \
+({ \
+ GUEST_ASSERT_EQ((b)->from, vmcb->save.br_from); \
+ GUEST_ASSERT_EQ((b)->to, vmcb->save.br_to); \
+}) \
+
+static void l2_guest_code(struct svm_test_data *svm)
+{
+ /* Record a branch, trigger save/restore, and make sure LBRs are intact */
+ RECORD_BRANCH(&l2_branch, "L2 branch");
+ GUEST_SYNC(true);
+ CHECK_BRANCH_MSRS(&l2_branch);
+ vmmcall();
+}
+
+static void l1_guest_code(struct svm_test_data *svm, bool nested_lbrv)
+{
+ unsigned long l2_guest_stack[L2_GUEST_STACK_SIZE];
+ struct vmcb *vmcb = svm->vmcb;
+ struct lbr_branch l1_branch;
+
+ /* Record a branch, trigger save/restore, and make sure LBRs are intact */
+ RECORD_BRANCH(&l1_branch, "L1 branch");
+ GUEST_SYNC(true);
+ CHECK_BRANCH_MSRS(&l1_branch);
+
+ /* Run L2, which will also do the same */
+ generic_svm_setup(svm, l2_guest_code,
+ &l2_guest_stack[L2_GUEST_STACK_SIZE]);
+
+ if (nested_lbrv)
+ vmcb->control.virt_ext = LBR_CTL_ENABLE_MASK;
+ else
+ vmcb->control.virt_ext &= ~LBR_CTL_ENABLE_MASK;
+
+ run_guest(vmcb, svm->vmcb_gpa);
+ GUEST_ASSERT(svm->vmcb->control.exit_code == SVM_EXIT_VMMCALL);
+
+ /* Trigger save/restore one more time before checking, just for kicks */
+ GUEST_SYNC(true);
+
+ /*
+ * If LBR_CTL_ENABLE is set, L1 and L2 should have separate LBR MSRs, so
+ * expect L1's LBRs to remain intact and L2 LBRs to be in the VMCB.
+ * Otherwise, the MSRs are shared between L1 & L2 so expect L2's LBRs.
+ */
+ if (nested_lbrv) {
+ CHECK_BRANCH_MSRS(&l1_branch);
+ CHECK_BRANCH_VMCB(&l2_branch, vmcb);
+ } else {
+ CHECK_BRANCH_MSRS(&l2_branch);
+ }
+ GUEST_DONE();
+}
+
+void test_lbrv_nested_state(bool nested_lbrv)
+{
+ struct kvm_x86_state *state = NULL;
+ struct kvm_vcpu *vcpu;
+ vm_vaddr_t svm_gva;
+ struct kvm_vm *vm;
+ struct ucall uc;
+
+ pr_info("Testing with nested LBRV %s\n", nested_lbrv ? "enabled" : "disabled");
+
+ vm = vm_create_with_one_vcpu(&vcpu, l1_guest_code);
+ vcpu_alloc_svm(vm, &svm_gva);
+ vcpu_args_set(vcpu, 2, svm_gva, nested_lbrv);
+
+ for (;;) {
+ vcpu_run(vcpu);
+ TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_IO);
+ switch (get_ucall(vcpu, &uc)) {
+ case UCALL_SYNC:
+ /* Save the vCPU state and restore it in a new VM on sync */
+ pr_info("Guest triggered save/restore.\n");
+ state = vcpu_save_state(vcpu);
+ kvm_vm_release(vm);
+ vcpu = vm_recreate_with_one_vcpu(vm);
+ vcpu_load_state(vcpu, state);
+ break;
+ case UCALL_ABORT:
+ REPORT_GUEST_ASSERT(uc);
+ /* NOT REACHED */
+ case UCALL_DONE:
+ goto done;
+ case UCALL_PRINTF:
+ pr_info("%s", uc.buffer);
+ break;
+ default:
+ TEST_FAIL("Unknown ucall %lu", uc.cmd);
+ }
+ }
+done:
+ if (state)
+ kvm_x86_state_cleanup(state);
+ kvm_vm_free(vm);
+}
+
+int main(int argc, char *argv[])
+{
+ TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_SVM));
+ TEST_REQUIRE(kvm_is_lbrv_enabled());
+
+ test_lbrv_nested_state(/*nested_lbrv=*/false);
+ test_lbrv_nested_state(/*nested_lbrv=*/true);
+
+ return 0;
+}
--
2.51.2.1041.gc1ab5b90ca-goog