Re: [PATCH v12 44/45] KVM: selftests: Update private_mem_conversions_test for in-place conversions

[Fuad Tabba]

On Mon, 31 Aug 2026 at 01:25, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
>
> Update private_mem_conversions_test for in-place conversions. In-place
> conversions support is detected in selftests with kvm_has_gmem_attributes.
>
> With in-place conversions, specifying userspace_addr from some other memory
> provider that is not the guest_memfd associated with the memslot is a user
> error, since KVM will only use both shared and private memory from the
> guest_memfd.
>
> Hence, when kvm_has_gmem_attributes, only test in-place conversions with
> single backing, where guest_memfd provides both shared and private memory.
>
> For single backing, guest_memfd must be created with
> GUEST_MEMFD_FLAG_MMAP. Initialize the guest_memfd as shared to align with
> how memory would default to shared when shared/private state was tracked at
> the VM level (the test expects this, it was written for
> non-in-place-conversions).
>
> When handling a hypercall to set attributes, use
> vm_mem_set_memory_attributes() to send the ioctl to the guest_memfd instead
> of the VM.
>
> When testing in-place conversions (single-backing), don't allow the user to
> configure src_type, since src_type will be ignored. Don't use src_type to
> determine alignment for rounding up per-cpu test memory size, since
> guest_memfd's backing page size is always the system PAGE_SIZE.
>
> Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>

Cheers,
/fuad


> ---
>  .../kvm/x86/private_mem_conversions_test.c         | 57 +++++++++++++++++-----
>  1 file changed, 44 insertions(+), 13 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/x86/private_mem_conversions_test.c b/tools/testing/selftests/kvm/x86/private_mem_conversions_test.c
> index cf50e9a332c5b..33b610383693b 100644
> --- a/tools/testing/selftests/kvm/x86/private_mem_conversions_test.c
> +++ b/tools/testing/selftests/kvm/x86/private_mem_conversions_test.c
> @@ -306,9 +306,12 @@ static void handle_exit_hypercall(struct kvm_vcpu *vcpu)
>         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);
> +       if (set_attributes) {
> +               u64 attrs = map_shared ? 0 : KVM_MEMORY_ATTRIBUTE_PRIVATE;
> +
> +               vm_mem_set_memory_attributes(vm, gpa, size, attrs);
> +       }
> +
>         run->hypercall.ret = 0;
>  }
>
> @@ -352,8 +355,21 @@ static void *__test_mem_conversions(void *__vcpu)
>                                 size_t nr_bytes = min_t(size_t, vm->page_size, size - i);
>                                 u8 *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);
> +                               if (kvm_has_gmem_attributes &&
> +                                   uc.args[0] == SYNC_PRIVATE) {
> +                                       TEST_EXPECT_SIGBUS(READ_ONCE(*hva));
> +                               } else {
> +                                       /*
> +                                        * If not testing in-place conversion
> +                                        * (dual backing), the host should
> +                                        * always observe shared data, since the
> +                                        * shared memory is a separate page.
> +                                        *
> +                                        * For SYNC_SHARED, test that the host
> +                                        * can see shared memory.
> +                                        */
> +                                       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)
> @@ -369,20 +385,29 @@ static void *__test_mem_conversions(void *__vcpu)
>         }
>  }
>
> +/* Align each vCPU's chunk of memory naturally to the size of the backing store. */
> +static size_t compute_per_cpu_size(enum vm_mem_backing_src_type src_type)
> +{
> +       size_t alignment;
> +
> +       if (kvm_has_gmem_attributes)
> +               alignment = getpagesize();
> +       else
> +               alignment = get_backing_src_pagesz(src_type);
> +
> +       return align_up(PER_CPU_DATA_SIZE, max_t(size_t, SZ_2M, alignment));
> +}
> +
>  static void test_mem_conversions(enum vm_mem_backing_src_type src_type, u32 nr_vcpus,
>                                  u32 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 per_cpu_size = compute_per_cpu_size(src_type);
>         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];
>         struct kvm_vm *vm;
> +       u64 gmem_flags;
>         int memfd, i;
>
>         const struct vm_shape shape = {
> @@ -397,12 +422,16 @@ static void test_mem_conversions(enum vm_mem_backing_src_type src_type, u32 nr_v
>
>         vm_enable_cap(vm, KVM_CAP_EXIT_HYPERCALL, (1 << KVM_HC_MAP_GPA_RANGE));
>
> -       memfd = vm_create_guest_memfd(vm, memfd_size, 0);
> +       gmem_flags = 0;
> +       if (kvm_has_gmem_attributes)
> +               gmem_flags = GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_INIT_SHARED;
> +
> +       memfd = vm_create_guest_memfd(vm, memfd_size, gmem_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_GUEST_MEMFD, memfd, slot_size * i, 0);
> +                          KVM_MEM_GUEST_MEMFD, memfd, slot_size * i, gmem_flags);
>
>         for (i = 0; i < nr_vcpus; i++) {
>                 gpa_t gpa =  BASE_DATA_GPA + i * per_cpu_size;
> @@ -462,6 +491,8 @@ int main(int argc, char *argv[])
>         while ((opt = getopt(argc, argv, "hm:s:n:")) != -1) {
>                 switch (opt) {
>                 case 's':
> +                       TEST_ASSERT(!kvm_has_gmem_attributes,
> +                                   "src_type is only configurable when testing without in-place conversion");
>                         src_type = parse_backing_src_type(optarg);
>                         break;
>                 case 'n':
>
> --
> 2.55.0.897.gb25b4bd76c-goog
>
>