Minimal test for guest_memfd to test that when memory is marked shared
in a VM, the host can read and write to it via an mmap()ed address,
and the guest can also read and write to it.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>

---
 tools/testing/selftests/kvm/Makefile          |   1 +
 .../selftests/kvm/guest_memfd_sharing_test.c  | 160 ++++++++++++++++++
 2 files changed, 161 insertions(+)
 create mode 100644 tools/testing/selftests/kvm/guest_memfd_sharing_test.c

diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index b3b7e83f39fc..3c1f35456bfc 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -135,6 +135,7 @@ TEST_GEN_PROGS_x86_64 += dirty_log_test
 TEST_GEN_PROGS_x86_64 += dirty_log_perf_test
 TEST_GEN_PROGS_x86_64 += guest_memfd_test
 TEST_GEN_PROGS_x86_64 += guest_memfd_hugetlb_reporting_test
+TEST_GEN_PROGS_x86_64 += guest_memfd_sharing_test
 TEST_GEN_PROGS_x86_64 += guest_print_test
 TEST_GEN_PROGS_x86_64 += hardware_disable_test
 TEST_GEN_PROGS_x86_64 += kvm_create_max_vcpus
diff --git a/tools/testing/selftests/kvm/guest_memfd_sharing_test.c b/tools/testing/selftests/kvm/guest_memfd_sharing_test.c
new file mode 100644
index 000000000000..fef5a73e5053
--- /dev/null
+++ b/tools/testing/selftests/kvm/guest_memfd_sharing_test.c
@@ -0,0 +1,160 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Minimal test for guest_memfd to test that when memory is marked shared in a
+ * VM, the host can read and write to it via an mmap()ed address, and the guest
+ * can also read and write to it.
+ *
+ * Copyright (c) 2024, Google LLC.
+ */
+#include <string.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#include "test_util.h"
+#include "kvm_util.h"
+#include "ucall_common.h"
+
+#define GUEST_MEMFD_SHARING_TEST_SLOT 10
+#define GUEST_MEMFD_SHARING_TEST_GPA 0x50000000ULL
+#define GUEST_MEMFD_SHARING_TEST_GVA 0x90000000ULL
+#define GUEST_MEMFD_SHARING_TEST_OFFSET 0
+#define GUEST_MEMFD_SHARING_TEST_GUEST_TO_HOST_VALUE 0x11
+#define GUEST_MEMFD_SHARING_TEST_HOST_TO_GUEST_VALUE 0x22
+
+static void guest_code(int page_size)
+{
+	char *mem;
+	int i;
+
+	mem = (char *)GUEST_MEMFD_SHARING_TEST_GVA;
+
+	for (i = 0; i < page_size; ++i) {
+		GUEST_ASSERT_EQ(mem[i], GUEST_MEMFD_SHARING_TEST_HOST_TO_GUEST_VALUE);
+	}
+
+	memset(mem, GUEST_MEMFD_SHARING_TEST_GUEST_TO_HOST_VALUE, page_size);
+
+	GUEST_DONE();
+}
+
+int run_test(struct kvm_vcpu *vcpu, void *hva, int page_size)
+{
+	struct ucall uc;
+	uint64_t uc_cmd;
+
+	memset(hva, GUEST_MEMFD_SHARING_TEST_HOST_TO_GUEST_VALUE, page_size);
+	vcpu_args_set(vcpu, 1, page_size);
+
+	/* Reset vCPU to guest_code every time run_test is called. */
+	vcpu_arch_set_entry_point(vcpu, guest_code);
+
+	vcpu_run(vcpu);
+	uc_cmd = get_ucall(vcpu, &uc);
+
+	if (uc_cmd == UCALL_ABORT) {
+		REPORT_GUEST_ASSERT(uc);
+		return 1;
+	} else if (uc_cmd == UCALL_DONE) {
+		char *mem;
+		int i;
+
+		mem = hva;
+		for (i = 0; i < page_size; ++i)
+			TEST_ASSERT_EQ(mem[i], GUEST_MEMFD_SHARING_TEST_GUEST_TO_HOST_VALUE);
+
+		return 0;
+	} else {
+		TEST_FAIL("Unknown ucall 0x%lx.", uc.cmd);
+		return 1;
+	}
+}
+
+void *add_memslot(struct kvm_vm *vm, int guest_memfd, size_t page_size,
+		  bool back_shared_memory_with_guest_memfd)
+{
+	void *mem;
+
+	if (back_shared_memory_with_guest_memfd) {
+		mem = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED,
+			   guest_memfd, GUEST_MEMFD_SHARING_TEST_OFFSET);
+	} else {
+		mem = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
+			   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	}
+	TEST_ASSERT(mem != MAP_FAILED, "mmap should return valid address");
+
+	/*
+	 * Setting up this memslot with a KVM_X86_SW_PROTECTED_VM marks all
+	 * offsets in the file as shared.
+	 */
+	vm_set_user_memory_region2(vm, GUEST_MEMFD_SHARING_TEST_SLOT,
+				   KVM_MEM_GUEST_MEMFD,
+				   GUEST_MEMFD_SHARING_TEST_GPA, page_size, mem,
+				   guest_memfd, GUEST_MEMFD_SHARING_TEST_OFFSET);
+
+	return mem;
+}
+
+void test_sharing(bool back_shared_memory_with_guest_memfd)
+{
+	const struct vm_shape shape = {
+		.mode = VM_MODE_DEFAULT,
+		.type = KVM_X86_SW_PROTECTED_VM,
+	};
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	size_t page_size;
+	int guest_memfd;
+	void *mem;
+
+	TEST_REQUIRE(kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SW_PROTECTED_VM));
+
+	vm = vm_create_shape_with_one_vcpu(shape, &vcpu, &guest_code);
+
+	page_size = getpagesize();
+
+	guest_memfd = vm_create_guest_memfd(vm, page_size, 0);
+
+	mem = add_memslot(vm, guest_memfd, page_size, back_shared_memory_with_guest_memfd);
+
+	virt_map(vm, GUEST_MEMFD_SHARING_TEST_GVA, GUEST_MEMFD_SHARING_TEST_GPA, 1);
+
+	run_test(vcpu, mem, page_size);
+
+	/* Toggle private flag of memory attributes and run the test again. */
+	if (back_shared_memory_with_guest_memfd) {
+		/*
+		 * Use MADV_REMOVE to release the backing guest_memfd memory
+		 * back to the system before it is used again. Test that this is
+		 * only necessary when guest_memfd is used to back shared
+		 * memory.
+		 */
+		madvise(mem, page_size, MADV_REMOVE);
+	}
+	vm_mem_set_private(vm, GUEST_MEMFD_SHARING_TEST_GPA, page_size);
+	vm_mem_set_shared(vm, GUEST_MEMFD_SHARING_TEST_GPA, page_size);
+
+	run_test(vcpu, mem, page_size);
+
+	kvm_vm_free(vm);
+	munmap(mem, page_size);
+	close(guest_memfd);
+}
+
+int main(int argc, char *argv[])
+{
+	/*
+	 * Confidence check that when guest_memfd is associated with a memslot
+	 * but only anonymous memory is used to back shared memory, sharing
+	 * memory between guest and host works as expected.
+	 */
+	test_sharing(false);
+
+	/*
+	 * Memory sharing should work as expected when shared memory is backed
+	 * with guest_memfd.
+	 */
+	test_sharing(true);
+
+	return 0;
+}
-- 
2.46.0.598.g6f2099f65c-goog