Re: [PATCH v8 28/46] KVM: selftests: Add support for mmap() on guest_memfd in core library

[Fuad Tabba]

On Fri, 19 Jun 2026 at 01:32, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
>
> Accept gmem_flags in vm_mem_add() to be able to create a guest_memfd within
> vm_mem_add().
>
> When vm_mem_add() is used to set up a guest_memfd for a memslot, set up the
> provided (or created) gmem_fd as the fd for the user memory region. This
> makes it available to be mmap()-ed from just like fds from other memory
> sources. mmap() from guest_memfd using the provided gmem_flags and
> gmem_offset.
>
> Add a kvm_slot_to_fd() helper to provide convenient access to the file
> descriptor of a memslot.
>
> Update existing callers of vm_mem_add() to pass 0 for gmem_flags to
> preserve existing behavior.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> [For guest_memfds, mmap() using gmem_offset instead of 0 all the time.]
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad

> ---
>  tools/testing/selftests/kvm/include/kvm_util.h     |  7 +++++-
>  tools/testing/selftests/kvm/lib/kvm_util.c         | 27 ++++++++++++----------
>  .../kvm/x86/private_mem_conversions_test.c         |  2 +-
>  3 files changed, 22 insertions(+), 14 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/include/kvm_util.h b/tools/testing/selftests/kvm/include/kvm_util.h
> index d4c104cb0418f..0cacf3698b259 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util.h
> @@ -700,7 +700,7 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
>                                  gpa_t gpa, u32 slot, u64 npages, u32 flags);
>  void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>                 gpa_t gpa, u32 slot, u64 npages, u32 flags,
> -               int gmem_fd, u64 gmem_offset);
> +               int gmem_fd, u64 gmem_offset, u64 gmem_flags);
>
>  #ifndef vm_arch_has_protected_memory
>  static inline bool vm_arch_has_protected_memory(struct kvm_vm *vm)
> @@ -732,6 +732,11 @@ void *addr_gva2hva(struct kvm_vm *vm, gva_t gva);
>  gpa_t addr_hva2gpa(struct kvm_vm *vm, void *hva);
>  void *addr_gpa2alias(struct kvm_vm *vm, gpa_t gpa);
>
> +static inline int kvm_slot_to_fd(struct kvm_vm *vm, u32 slot)
> +{
> +       return memslot2region(vm, slot)->fd;
> +}
> +
>  #ifndef vcpu_arch_put_guest
>  #define vcpu_arch_put_guest(mem, val) do { (mem) = (val); } while (0)
>  #endif
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index 9b482778f7379..d5bbc80b2bf1c 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -978,12 +978,13 @@ void vm_set_user_memory_region2(struct kvm_vm *vm, u32 slot, u32 flags,
>  /* FIXME: This thing needs to be ripped apart and rewritten. */
>  void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>                 gpa_t gpa, u32 slot, u64 npages, u32 flags,
> -               int gmem_fd, u64 gmem_offset)
> +               int gmem_fd, u64 gmem_offset, u64 gmem_flags)
>  {
>         int ret;
>         struct userspace_mem_region *region;
>         size_t backing_src_pagesz = get_backing_src_pagesz(src_type);
>         size_t mem_size = npages * vm->page_size;
> +       off_t mmap_offset = 0;
>         size_t alignment = 1;
>
>         TEST_REQUIRE_SET_USER_MEMORY_REGION2();
> @@ -1055,8 +1056,6 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>
>         if (flags & KVM_MEM_GUEST_MEMFD) {
>                 if (gmem_fd < 0) {
> -                       u32 gmem_flags = 0;
> -
>                         TEST_ASSERT(!gmem_offset,
>                                     "Offset must be zero when creating new guest_memfd");
>                         gmem_fd = vm_create_guest_memfd(vm, mem_size, gmem_flags);
> @@ -1077,13 +1076,17 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>         }
>
>         region->fd = -1;
> -       if (backing_src_is_shared(src_type))
> +       if (flags & KVM_MEM_GUEST_MEMFD && gmem_flags & GUEST_MEMFD_FLAG_MMAP) {
> +               region->fd = kvm_dup(gmem_fd);
> +               mmap_offset = gmem_offset;
> +       } else if (backing_src_is_shared(src_type)) {
>                 region->fd = kvm_memfd_alloc(region->mmap_size,
>                                              src_type == VM_MEM_SRC_SHARED_HUGETLB);
> +       }
>
> -       region->mmap_start = kvm_mmap(region->mmap_size, PROT_READ | PROT_WRITE,
> -                                     vm_mem_backing_src_alias(src_type)->flag,
> -                                     region->fd);
> +       region->mmap_start = __kvm_mmap(region->mmap_size, PROT_READ | PROT_WRITE,
> +                                       vm_mem_backing_src_alias(src_type)->flag,
> +                                       region->fd, mmap_offset);
>
>         TEST_ASSERT(!is_backing_src_hugetlb(src_type) ||
>                     region->mmap_start == align_ptr_up(region->mmap_start, backing_src_pagesz),
> @@ -1129,10 +1132,10 @@ void vm_mem_add(struct kvm_vm *vm, enum vm_mem_backing_src_type src_type,
>
>         /* If shared memory, create an alias. */
>         if (region->fd >= 0) {
> -               region->mmap_alias = kvm_mmap(region->mmap_size,
> -                                             PROT_READ | PROT_WRITE,
> -                                             vm_mem_backing_src_alias(src_type)->flag,
> -                                             region->fd);
> +               region->mmap_alias = __kvm_mmap(region->mmap_size,
> +                                               PROT_READ | PROT_WRITE,
> +                                               vm_mem_backing_src_alias(src_type)->flag,
> +                                               region->fd, mmap_offset);
>
>                 /* Align host alias address */
>                 region->host_alias = align_ptr_up(region->mmap_alias, alignment);
> @@ -1143,7 +1146,7 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
>                                  enum vm_mem_backing_src_type src_type,
>                                  gpa_t gpa, u32 slot, u64 npages, u32 flags)
>  {
> -       vm_mem_add(vm, src_type, gpa, slot, npages, flags, -1, 0);
> +       vm_mem_add(vm, src_type, gpa, slot, npages, flags, -1, 0, 0);
>  }
>
>  /*
> 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 1d2f5d4fd45d7..861baff201e78 100644
> --- a/tools/testing/selftests/kvm/x86/private_mem_conversions_test.c
> +++ b/tools/testing/selftests/kvm/x86/private_mem_conversions_test.c
> @@ -399,7 +399,7 @@ static void test_mem_conversions(enum vm_mem_backing_src_type src_type, u32 nr_v
>         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);
> +                          KVM_MEM_GUEST_MEMFD, memfd, slot_size * i, 0);
>
>         for (i = 0; i < nr_vcpus; i++) {
>                 gpa_t gpa =  BASE_DATA_GPA + i * per_cpu_size;
>
> --
> 2.55.0.rc0.738.g0c8ab3ebcc-goog
>
>