Re: [PATCH v8 45/46] KVM: selftests: Update private_mem_conversions_test to mmap() guest_memfd

[Fuad Tabba]

On Fri, 19 Jun 2026 at 01:32, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
>
> Update the private memory conversions selftest to also test conversions
> that are done "in-place" via per-guest_memfd memory attributes. In-place
> conversions require the host to be able to mmap() the guest_memfd so that
> the host and guest can share the same backing physical memory.
>
> This includes several updates, that are conditioned on the system
> supporting per-guest_memfd attributes (kvm_has_gmem_attributes):
>
> 1. Set up guest_memfd requesting MMAP and INIT_SHARED.
>
> 2. With in-place conversions, the host's mapping points directly to the
>    guest's memory. When the guest converts a region to private, host access
>    to that region is blocked. Update the test to expect a SIGBUS when
>    attempting to access the host virtual address (HVA) of private memory.
>
> 3. Use vm_mem_set_memory_attributes(), which chooses how to set memory
>    attributes based on whether kvm_has_gmem_attributes.
>
> Restrict the test to using VM_MEM_SRC_SHMEM because guest_memfd's required
> mmap() flags and page sizes happens to align with those of
> VM_MEM_SRC_SHMEM. As long as VM_MEM_SRC_SHMEM is used for src_type,
> vm_mem_add() works as intended.
>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad

> ---
>  .../kvm/x86/private_mem_conversions_test.c         | 44 ++++++++++++++++++----
>  1 file changed, 36 insertions(+), 8 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 289ad10063fca..4308c67952310 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 +355,20 @@ 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);
> +                               /*
> +                                * When using per-guest_memfd memory attributes,
> +                                * i.e. in-place conversion, host accesses will
> +                                * point at guest memory and should SIGBUS when
> +                                * guest memory is private.  When using per-VM
> +                                * attributes, i.e. separate backing for shared
> +                                * vs. private, the host should always observe
> +                                * the shared data.
> +                                */
> +                               if (kvm_has_gmem_attributes &&
> +                                   uc.args[0] == SYNC_PRIVATE)
> +                                       TEST_EXPECT_SIGBUS(READ_ONCE(*hva));
> +                               else
> +                                       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)
> @@ -382,6 +397,7 @@ static void test_mem_conversions(enum vm_mem_backing_src_type src_type, u32 nr_v
>         const size_t slot_size = memfd_size / nr_memslots;
>         struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
>         pthread_t threads[KVM_MAX_VCPUS];
> +       u64 gmem_flags;
>         struct kvm_vm *vm;
>         int memfd, i;
>
> @@ -397,12 +413,17 @@ 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);
> +       if (kvm_has_gmem_attributes)
> +               gmem_flags = GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_INIT_SHARED;
> +       else
> +               gmem_flags = 0;
> +
> +       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;
> @@ -452,17 +473,24 @@ static void usage(const char *cmd)
>
>  int main(int argc, char *argv[])
>  {
> -       enum vm_mem_backing_src_type src_type = DEFAULT_VM_MEM_SRC;
> +       enum vm_mem_backing_src_type src_type;
>         u32 nr_memslots = 1;
>         u32 nr_vcpus = 1;
>         int opt;
>
>         TEST_REQUIRE(kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SW_PROTECTED_VM));
>
> +       src_type = kvm_has_gmem_attributes ? VM_MEM_SRC_SHMEM :
> +                                            DEFAULT_VM_MEM_SRC;
> +
>         while ((opt = getopt(argc, argv, "hm:s:n:")) != -1) {
>                 switch (opt) {
>                 case 's':
>                         src_type = parse_backing_src_type(optarg);
> +                       TEST_ASSERT(!kvm_has_gmem_attributes ||
> +                                   src_type == VM_MEM_SRC_SHMEM,
> +                                   "Testing in-place conversions, only %s mem_type supported\n",
> +                                   vm_mem_backing_src_alias(VM_MEM_SRC_SHMEM)->name);
>                         break;
>                 case 'n':
>                         nr_vcpus = atoi_positive("nr_vcpus", optarg);
>
> --
> 2.55.0.rc0.738.g0c8ab3ebcc-goog
>
>