[PATCH v7 12/12] KVM: selftests: Test guest execution from direct map removed gmem

Roy, Patrick
2025-09-24T15:23:04Z

Add a selftest that loads itself into guest_memfd (via
GUEST_MEMFD_FLAG_MMAP) and triggers an MMIO exit when executed. This
exercises x86 MMIO emulation code inside KVM for guest_memfd-backed
memslots where the guest_memfd folios are direct map removed.
Particularly, it validates that x86 MMIO emulation code (guest page
table walks + instruction fetch) correctly accesses gmem through the VMA
that's been reflected into the memslot's userspace_addr field (instead
of trying to do direct map accesses).

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
---
.../selftests/kvm/set_memory_region_test.c | 50 +++++++++++++++++--
1 file changed, 46 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/kvm/set_memory_region_test.c b/tools/testing/selftests/kvm/set_memory_region_test.c
index ce3ac0fd6dfb..cb3bc642d376 100644
--- a/tools/testing/selftests/kvm/set_memory_region_test.c
+++ b/tools/testing/selftests/kvm/set_memory_region_test.c
@@ -603,6 +603,41 @@ static void test_mmio_during_vectoring(void)

kvm_vm_free(vm);
}
+
+static void guest_code_trigger_mmio(void)
+{
+ /*
+ * Read some GPA that is not backed by a memslot. KVM consider this
+ * as MMIO and tell userspace to emulate the read.
+ */
+ READ_ONCE(*((uint64_t *)MEM_REGION_GPA));
+
+ GUEST_DONE();
+}
+
+static void test_guest_memfd_mmio(void)
+{
+ struct kvm_vm *vm;
+ struct kvm_vcpu *vcpu;
+ struct vm_shape shape = {
+ .mode = VM_MODE_DEFAULT,
+ .src_type = VM_MEM_SRC_GUEST_MEMFD_NO_DIRECT_MAP,
+ };
+ pthread_t vcpu_thread;
+
+ pr_info("Testing MMIO emulation for instructions in gmem\n");
+
+ vm = __vm_create_shape_with_one_vcpu(shape, &vcpu, 0, guest_code_trigger_mmio);
+
+ virt_map(vm, MEM_REGION_GPA, MEM_REGION_GPA, 1);
+
+ pthread_create(&vcpu_thread, NULL, vcpu_worker, vcpu);
+
+ /* If the MMIO read was successfully emulated, the vcpu thread will exit */
+ pthread_join(vcpu_thread, NULL);
+
+ kvm_vm_free(vm);
+}
#endif

int main(int argc, char *argv[])
@@ -626,10 +661,17 @@ int main(int argc, char *argv[])
test_add_max_memory_regions();

#ifdef __x86_64__
- if (kvm_has_cap(KVM_CAP_GUEST_MEMFD) &&
- (kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SW_PROTECTED_VM))) {
- test_add_private_memory_region();
- test_add_overlapping_private_memory_regions();
+ if (kvm_has_cap(KVM_CAP_GUEST_MEMFD)) {
+ if (kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SW_PROTECTED_VM)) {
+ test_add_private_memory_region();
+ test_add_overlapping_private_memory_regions();
+ }
+
+ if (kvm_has_cap(KVM_CAP_GUEST_MEMFD_MMAP) &&
+ kvm_has_cap(KVM_CAP_GUEST_MEMFD_NO_DIRECT_MAP))
+ test_guest_memfd_mmio();
+ else
+ pr_info("Skipping tests requiring KVM_CAP_GUEST_MEMFD_MMAP | KVM_CAP_GUEST_MEMFD_NO_DIRECT_MAP");
} else {
pr_info("Skipping tests for KVM_MEM_GUEST_MEMFD memory regions\n");
}
--
2.51.0