On Wed Sep 24, 2025 at 3:22 PM UTC, Patrick Roy wrote:
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);
When I run this test on my minimal config in a nested VM I get:
[root@testvm:~]# /nix/store/xlxd60n7v1qfr6s5zxda410zrzdd0xc2-kselftests/bin/run_kselftest.sh -t kvm:set_memory_region_test TAP version 13 1..1 # timeout set to 120 # selftests: kvm: set_memory_region_test # Random seed: 0x6b8b4567 # Testing KVM_RUN with zero added memory regions # Testing MMIO during vectoring error handling # Allowed number of memory slots: 32764 # Adding slots 0..32763, each memory region with 2048K size # Testing MMIO emulation for instructions in gmem # ==== Test Assertion Failure ==== # lib/kvm_util.c:1118: region->mmap_start != MAP_FAILED # pid=614 tid=614 errno=19 - No such device # 1 0x0000000000407b02: vm_mem_add at ??:? # 2 0x000000000040a924: __vm_create at ??:? # 3 0x000000000040ab16: __vm_create_shape_with_one_vcpu at ??:? # 4 0x00000000004042cf: main at ??:? # 5 0x00007faa6b08a47d: ?? ??:0 # 6 0x00007faa6b08a538: ?? ??:0 # 7 0x0000000000404384: _start at ??:? # mmap() failed, rc: -1 errno: 19 (No such device)
Here's the kconfig I'm using (basically defconfig+KVM):
https://gist.githubusercontent.com/bjackman/4ea941ef5072606769211f3b000c8ed7...
Sorry I'm too ignorant about KVM to know what I'm missing here but I guess it's a missing TEST_REQUIRE()?