diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index a3bb36fb3cfc..b709a52d5cdb 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -81,6 +81,7 @@ TEST_GEN_PROGS_x86_64 += x86_64/monitor_mwait_test
 TEST_GEN_PROGS_x86_64 += x86_64/nested_exceptions_test
 TEST_GEN_PROGS_x86_64 += x86_64/platform_info_test
 TEST_GEN_PROGS_x86_64 += x86_64/pmu_event_filter_test
+TEST_GEN_PROGS_x86_64 += x86_64/private_mem_conversions_test
 TEST_GEN_PROGS_x86_64 += x86_64/set_boot_cpu_id
 TEST_GEN_PROGS_x86_64 += x86_64/set_sregs_test
 TEST_GEN_PROGS_x86_64 += x86_64/smaller_maxphyaddr_emulation_test
diff --git a/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c b/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
new file mode 100644
index 000000000000..be311944e90a
--- /dev/null
+++ b/tools/testing/selftests/kvm/x86_64/private_mem_conversions_test.c
@@ -0,0 +1,487 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2022, Google LLC.
+ */
+#define _GNU_SOURCE /* for program_invocation_short_name */
+#include <fcntl.h>
+#include <limits.h>
+#include <pthread.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+
+#include <linux/compiler.h>
+#include <linux/kernel.h>
+#include <linux/kvm_para.h>
+#include <linux/memfd.h>
+#include <linux/sizes.h>
+
+#include <test_util.h>
+#include <kvm_util.h>
+#include <processor.h>
+
+#define BASE_DATA_SLOT		10
+#define BASE_DATA_GPA		((uint64_t)(1ull << 32))
+#define PER_CPU_DATA_SIZE	((uint64_t)(SZ_2M + PAGE_SIZE))
+
+/* Horrific macro so that the line info is captured accurately :-( */
+#define memcmp_g(gpa, pattern,  size)								\
+do {												\
+	uint8_t *mem = (uint8_t *)gpa;								\
+	size_t i;										\
+												\
+	for (i = 0; i < size; i++)								\
+		__GUEST_ASSERT(mem[i] == pattern,						\
+			       "Guest expected 0x%x at offset %lu (gpa 0x%llx), got 0x%x",	\
+			       pattern, i, gpa + i, mem[i]);					\
+} while (0)
+
+static void memcmp_h(uint8_t *mem, uint64_t gpa, uint8_t pattern, size_t size)
+{
+	size_t i;
+
+	for (i = 0; i < size; i++)
+		TEST_ASSERT(mem[i] == pattern,
+			    "Host expected 0x%x at gpa 0x%lx, got 0x%x",
+			    pattern, gpa + i, mem[i]);
+}
+
+/*
+ * Run memory conversion tests with explicit conversion:
+ * Execute KVM hypercall to map/unmap gpa range which will cause userspace exit
+ * to back/unback private memory. Subsequent accesses by guest to the gpa range
+ * will not cause exit to userspace.
+ *
+ * Test memory conversion scenarios with following steps:
+ * 1) Access private memory using private access and verify that memory contents
+ *   are not visible to userspace.
+ * 2) Convert memory to shared using explicit conversions and ensure that
+ *   userspace is able to access the shared regions.
+ * 3) Convert memory back to private using explicit conversions and ensure that
+ *   userspace is again not able to access converted private regions.
+ */
+
+#define GUEST_STAGE(o, s) { .offset = o, .size = s }
+
+enum ucall_syncs {
+	SYNC_SHARED,
+	SYNC_PRIVATE,
+};
+
+static void guest_sync_shared(uint64_t gpa, uint64_t size,
+			      uint8_t current_pattern, uint8_t new_pattern)
+{
+	GUEST_SYNC5(SYNC_SHARED, gpa, size, current_pattern, new_pattern);
+}
+
+static void guest_sync_private(uint64_t gpa, uint64_t size, uint8_t pattern)
+{
+	GUEST_SYNC4(SYNC_PRIVATE, gpa, size, pattern);
+}
+
+/* Arbitrary values, KVM doesn't care about the attribute flags. */
+#define MAP_GPA_SET_ATTRIBUTES	BIT(0)
+#define MAP_GPA_SHARED		BIT(1)
+#define MAP_GPA_DO_FALLOCATE	BIT(2)
+
+static void guest_map_mem(uint64_t gpa, uint64_t size, bool map_shared,
+			  bool do_fallocate)
+{
+	uint64_t flags = MAP_GPA_SET_ATTRIBUTES;
+
+	if (map_shared)
+		flags |= MAP_GPA_SHARED;
+	if (do_fallocate)
+		flags |= MAP_GPA_DO_FALLOCATE;
+	kvm_hypercall_map_gpa_range(gpa, size, flags);
+}
+
+static void guest_map_shared(uint64_t gpa, uint64_t size, bool do_fallocate)
+{
+	guest_map_mem(gpa, size, true, do_fallocate);
+}
+
+static void guest_map_private(uint64_t gpa, uint64_t size, bool do_fallocate)
+{
+	guest_map_mem(gpa, size, false, do_fallocate);
+}
+
+struct {
+	uint64_t offset;
+	uint64_t size;
+} static const test_ranges[] = {
+	GUEST_STAGE(0, PAGE_SIZE),
+	GUEST_STAGE(0, SZ_2M),
+	GUEST_STAGE(PAGE_SIZE, PAGE_SIZE),
+	GUEST_STAGE(PAGE_SIZE, SZ_2M),
+	GUEST_STAGE(SZ_2M, PAGE_SIZE),
+};
+
+static void guest_test_explicit_conversion(uint64_t base_gpa, bool do_fallocate)
+{
+	const uint8_t def_p = 0xaa;
+	const uint8_t init_p = 0xcc;
+	uint64_t j;
+	int i;
+
+	/* Memory should be shared by default. */
+	memset((void *)base_gpa, def_p, PER_CPU_DATA_SIZE);
+	memcmp_g(base_gpa, def_p, PER_CPU_DATA_SIZE);
+	guest_sync_shared(base_gpa, PER_CPU_DATA_SIZE, def_p, init_p);
+
+	memcmp_g(base_gpa, init_p, PER_CPU_DATA_SIZE);
+
+	for (i = 0; i < ARRAY_SIZE(test_ranges); i++) {
+		uint64_t gpa = base_gpa + test_ranges[i].offset;
+		uint64_t size = test_ranges[i].size;
+		uint8_t p1 = 0x11;
+		uint8_t p2 = 0x22;
+		uint8_t p3 = 0x33;
+		uint8_t p4 = 0x44;
+
+		/*
+		 * Set the test region to pattern one to differentiate it from
+		 * the data range as a whole (contains the initial pattern).
+		 */
+		memset((void *)gpa, p1, size);
+
+		/*
+		 * Convert to private, set and verify the private data, and
+		 * then verify that the rest of the data (map shared) still
+		 * holds the initial pattern, and that the host always sees the
+		 * shared memory (initial pattern).  Unlike shared memory,
+		 * punching a hole in private memory is destructive, i.e.
+		 * previous values aren't guaranteed to be preserved.
+		 */
+		guest_map_private(gpa, size, do_fallocate);
+
+		if (size > PAGE_SIZE) {
+			memset((void *)gpa, p2, PAGE_SIZE);
+			goto skip;
+		}
+
+		memset((void *)gpa, p2, size);
+		guest_sync_private(gpa, size, p1);
+
+		/*
+		 * Verify that the private memory was set to pattern two, and
+		 * that shared memory still holds the initial pattern.
+		 */
+		memcmp_g(gpa, p2, size);
+		if (gpa > base_gpa)
+			memcmp_g(base_gpa, init_p, gpa - base_gpa);
+		if (gpa + size < base_gpa + PER_CPU_DATA_SIZE)
+			memcmp_g(gpa + size, init_p,
+				 (base_gpa + PER_CPU_DATA_SIZE) - (gpa + size));
+
+		/*
+		 * Convert odd-number page frames back to shared to verify KVM
+		 * also correctly handles holes in private ranges.
+		 */
+		for (j = 0; j < size; j += PAGE_SIZE) {
+			if ((j >> PAGE_SHIFT) & 1) {
+				guest_map_shared(gpa + j, PAGE_SIZE, do_fallocate);
+				guest_sync_shared(gpa + j, PAGE_SIZE, p1, p3);
+
+				memcmp_g(gpa + j, p3, PAGE_SIZE);
+			} else {
+				guest_sync_private(gpa + j, PAGE_SIZE, p1);
+			}
+		}
+
+skip:
+		/*
+		 * Convert the entire region back to shared, explicitly write
+		 * pattern three to fill in the even-number frames before
+		 * asking the host to verify (and write pattern four).
+		 */
+		guest_map_shared(gpa, size, do_fallocate);
+		memset((void *)gpa, p3, size);
+		guest_sync_shared(gpa, size, p3, p4);
+		memcmp_g(gpa, p4, size);
+
+		/* Reset the shared memory back to the initial pattern. */
+		memset((void *)gpa, init_p, size);
+
+		/*
+		 * Free (via PUNCH_HOLE) *all* private memory so that the next
+		 * iteration starts from a clean slate, e.g. with respect to
+		 * whether or not there are pages/folios in guest_mem.
+		 */
+		guest_map_shared(base_gpa, PER_CPU_DATA_SIZE, true);
+	}
+}
+
+static void guest_punch_hole(uint64_t gpa, uint64_t size)
+{
+	/* "Mapping" memory shared via fallocate() is done via PUNCH_HOLE. */
+	uint64_t flags = MAP_GPA_SHARED | MAP_GPA_DO_FALLOCATE;
+
+	kvm_hypercall_map_gpa_range(gpa, size, flags);
+}
+
+/*
+ * Test that PUNCH_HOLE actually frees memory by punching holes without doing a
+ * proper conversion.  Freeing (PUNCH_HOLE) should zap SPTEs, and reallocating
+ * (subsequent fault) should zero memory.
+ */
+static void guest_test_punch_hole(uint64_t base_gpa, bool precise)
+{
+	const uint8_t init_p = 0xcc;
+	int i;
+
+	/*
+	 * Convert the entire range to private, this testcase is all about
+	 * punching holes in guest_memfd, i.e. shared mappings aren't needed.
+	 */
+	guest_map_private(base_gpa, PER_CPU_DATA_SIZE, false);
+
+	for (i = 0; i < ARRAY_SIZE(test_ranges); i++) {
+		uint64_t gpa = base_gpa + test_ranges[i].offset;
+		uint64_t size = test_ranges[i].size;
+
+		/*
+		 * Free all memory before each iteration, even for the !precise
+		 * case where the memory will be faulted back in.  Freeing and
+		 * reallocating should obviously work, and freeing all memory
+		 * minimizes the probability of cross-testcase influence.
+		 */
+		guest_punch_hole(base_gpa, PER_CPU_DATA_SIZE);
+
+		/* Fault-in and initialize memory, and verify the pattern. */
+		if (precise) {
+			memset((void *)gpa, init_p, size);
+			memcmp_g(gpa, init_p, size);
+		} else {
+			memset((void *)base_gpa, init_p, PER_CPU_DATA_SIZE);
+			memcmp_g(base_gpa, init_p, PER_CPU_DATA_SIZE);
+		}
+
+		/*
+		 * Punch a hole at the target range and verify that reads from
+		 * the guest succeed and return zeroes.
+		 */
+		guest_punch_hole(gpa, size);
+		memcmp_g(gpa, 0, size);
+	}
+}
+
+static void guest_code(uint64_t base_gpa)
+{
+	/*
+	 * Run the conversion test twice, with and without doing fallocate() on
+	 * the guest_memfd backing when converting between shared and private.
+	 */
+	guest_test_explicit_conversion(base_gpa, false);
+	guest_test_explicit_conversion(base_gpa, true);
+
+	/*
+	 * Run the PUNCH_HOLE test twice too, once with the entire guest_memfd
+	 * faulted in, once with only the target range faulted in.
+	 */
+	guest_test_punch_hole(base_gpa, false);
+	guest_test_punch_hole(base_gpa, true);
+	GUEST_DONE();
+}
+
+static void handle_exit_hypercall(struct kvm_vcpu *vcpu)
+{
+	struct kvm_run *run = vcpu->run;
+	uint64_t gpa = run->hypercall.args[0];
+	uint64_t size = run->hypercall.args[1] * PAGE_SIZE;
+	bool set_attributes = run->hypercall.args[2] & MAP_GPA_SET_ATTRIBUTES;
+	bool map_shared = run->hypercall.args[2] & MAP_GPA_SHARED;
+	bool do_fallocate = run->hypercall.args[2] & MAP_GPA_DO_FALLOCATE;
+	struct kvm_vm *vm = vcpu->vm;
+
+	TEST_ASSERT(run->hypercall.nr == KVM_HC_MAP_GPA_RANGE,
+		    "Wanted MAP_GPA_RANGE (%u), got '%llu'",
+		    KVM_HC_MAP_GPA_RANGE, run->hypercall.nr);
+
+	if (do_fallocate)
+		vm_guest_mem_fallocate(vm, gpa, size, map_shared);
+
+	if (set_attributes)
+		vm_set_memory_attributes(vm, gpa, size,
+					 map_shared ? 0 : KVM_MEMORY_ATTRIBUTE_PRIVATE);
+	run->hypercall.ret = 0;
+}
+
+static bool run_vcpus;
+
+static void *__test_mem_conversions(void *__vcpu)
+{
+	struct kvm_vcpu *vcpu = __vcpu;
+	struct kvm_run *run = vcpu->run;
+	struct kvm_vm *vm = vcpu->vm;
+	struct ucall uc;
+
+	while (!READ_ONCE(run_vcpus))
+		;
+
+	for ( ;; ) {
+		vcpu_run(vcpu);
+
+		if (run->exit_reason == KVM_EXIT_HYPERCALL) {
+			handle_exit_hypercall(vcpu);
+			continue;
+		}
+
+		TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
+			    "Wanted KVM_EXIT_IO, got exit reason: %u (%s)",
+			    run->exit_reason, exit_reason_str(run->exit_reason));
+
+		switch (get_ucall(vcpu, &uc)) {
+		case UCALL_ABORT:
+			REPORT_GUEST_ASSERT(uc);
+		case UCALL_SYNC: {
+			uint64_t gpa  = uc.args[1];
+			size_t size = uc.args[2];
+			size_t i;
+
+			TEST_ASSERT(uc.args[0] == SYNC_SHARED ||
+				    uc.args[0] == SYNC_PRIVATE,
+				    "Unknown sync command '%ld'", uc.args[0]);
+
+			for (i = 0; i < size; i += vm->page_size) {
+				size_t nr_bytes = min_t(size_t, vm->page_size, size - i);
+				uint8_t *hva = addr_gpa2hva(vm, gpa + i);
+
+				/* In all cases, the host should observe the shared data. */
+				memcmp_h(hva, gpa + i, uc.args[3], nr_bytes);
+
+				/* For shared, write the new pattern to guest memory. */
+				if (uc.args[0] == SYNC_SHARED)
+					memset(hva, uc.args[4], nr_bytes);
+			}
+			break;
+		}
+		case UCALL_DONE:
+			return NULL;
+		default:
+			TEST_FAIL("Unknown ucall 0x%lx.", uc.cmd);
+		}
+	}
+}
+
+static void test_mem_conversions(enum vm_mem_backing_src_type src_type, uint32_t nr_vcpus,
+				 uint32_t nr_memslots)
+{
+	/*
+	 * Allocate enough memory so that each vCPU's chunk of memory can be
+	 * naturally aligned with respect to the size of the backing store.
+	 */
+	const size_t alignment = max_t(size_t, SZ_2M, get_backing_src_pagesz(src_type));
+	const size_t per_cpu_size = align_up(PER_CPU_DATA_SIZE, alignment);
+	const size_t memfd_size = per_cpu_size * nr_vcpus;
+	const size_t slot_size = memfd_size / nr_memslots;
+	struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
+	pthread_t threads[KVM_MAX_VCPUS];
+	uint64_t memfd_flags;
+	struct kvm_vm *vm;
+	int memfd, i, r;
+
+	const struct vm_shape shape = {
+		.mode = VM_MODE_DEFAULT,
+		.type = KVM_X86_SW_PROTECTED_VM,
+	};
+
+	TEST_ASSERT(slot_size * nr_memslots == memfd_size,
+		    "The memfd size (0x%lx) needs to be cleanly divisible by the number of memslots (%u)",
+		    memfd_size, nr_memslots);
+	vm = __vm_create_with_vcpus(shape, nr_vcpus, 0, guest_code, vcpus);
+
+	vm_enable_cap(vm, KVM_CAP_EXIT_HYPERCALL, (1 << KVM_HC_MAP_GPA_RANGE));
+
+	if (backing_src_can_be_huge(src_type))
+		memfd_flags = KVM_GUEST_MEMFD_ALLOW_HUGEPAGE;
+	else
+		memfd_flags = 0;
+	memfd = vm_create_guest_memfd(vm, memfd_size, memfd_flags);
+
+	for (i = 0; i < nr_memslots; i++)
+		vm_mem_add(vm, src_type, BASE_DATA_GPA + slot_size * i,
+			   BASE_DATA_SLOT + i, slot_size / vm->page_size,
+			   KVM_MEM_PRIVATE, memfd, slot_size * i);
+
+	for (i = 0; i < nr_vcpus; i++) {
+		uint64_t gpa =  BASE_DATA_GPA + i * per_cpu_size;
+
+		vcpu_args_set(vcpus[i], 1, gpa);
+
+		/*
+		 * Map only what is needed so that an out-of-bounds access
+		 * results #PF => SHUTDOWN instead of data corruption.
+		 */
+		virt_map(vm, gpa, gpa, PER_CPU_DATA_SIZE / vm->page_size);
+
+		pthread_create(&threads[i], NULL, __test_mem_conversions, vcpus[i]);
+	}
+
+	WRITE_ONCE(run_vcpus, true);
+
+	for (i = 0; i < nr_vcpus; i++)
+		pthread_join(threads[i], NULL);
+
+	kvm_vm_free(vm);
+
+	/*
+	 * Allocate and free memory from the guest_memfd after closing the VM
+	 * fd.  The guest_memfd is gifted a reference to its owning VM, i.e.
+	 * should prevent the VM from being fully destroyed until the last
+	 * reference to the guest_memfd is also put.
+	 */
+	r = fallocate(memfd, FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE, 0, memfd_size);
+	TEST_ASSERT(!r, __KVM_SYSCALL_ERROR("fallocate()", r));
+
+	r = fallocate(memfd, FALLOC_FL_KEEP_SIZE, 0, memfd_size);
+	TEST_ASSERT(!r, __KVM_SYSCALL_ERROR("fallocate()", r));
+}
+
+static void usage(const char *cmd)
+{
+	puts("");
+	printf("usage: %s [-h] [-m nr_memslots] [-s mem_type] [-n nr_vcpus]\n", cmd);
+	puts("");
+	backing_src_help("-s");
+	puts("");
+	puts(" -n: specify the number of vcpus (default: 1)");
+	puts("");
+	puts(" -m: specify the number of memslots (default: 1)");
+	puts("");
+}
+
+int main(int argc, char *argv[])
+{
+	enum vm_mem_backing_src_type src_type = DEFAULT_VM_MEM_SRC;
+	uint32_t nr_memslots = 1;
+	uint32_t nr_vcpus = 1;
+	int opt;
+
+	TEST_REQUIRE(kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SW_PROTECTED_VM));
+
+	while ((opt = getopt(argc, argv, "hm:s:n:")) != -1) {
+		switch (opt) {
+		case 's':
+			src_type = parse_backing_src_type(optarg);
+			break;
+		case 'n':
+			nr_vcpus = atoi_positive("nr_vcpus", optarg);
+			break;
+		case 'm':
+			nr_memslots = atoi_positive("nr_memslots", optarg);
+			break;
+		case 'h':
+		default:
+			usage(argv[0]);
+			exit(0);
+		}
+	}
+
+	test_mem_conversions(src_type, nr_vcpus, nr_memslots);
+
+	return 0;
+}
