Re: [PATCH v8 33/46] KVM: selftests: Test conversion precision in 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>
>
> The existing guest_memfd conversion tests only use single-page memory
> regions. This provides no coverage for multi-page guest_memfd objects,
> specifically whether KVM correctly handles the page index for conversion
> operations. An incorrect implementation could, for example, always operate
> on the first page regardless of the index provided.
>
> Add a new test case to verify that conversions between private and shared
> memory correctly target the specified page within a multi-page guest_memfd.
>
> This test also verifies the precision of memory conversions by converting a
> single page an then iterating through all other pages ensure they remain in
> their original state.
>
> To support this test, add a new GMEM_CONVERSION_MULTIPAGE_TEST_INIT_SHARED
> macro that handles setting up and tearing down the VM for each page
> iteration. The teardown logic is adjusted to prevent a double-free in this
> new scenario.
>
> 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/guest_memfd_conversions_test.c         | 66 ++++++++++++++++++++++
>  1 file changed, 66 insertions(+)
>
> diff --git a/tools/testing/selftests/kvm/x86/guest_memfd_conversions_test.c b/tools/testing/selftests/kvm/x86/guest_memfd_conversions_test.c
> index 5b070d3374eae..8e17d5c08aeb8 100644
> --- a/tools/testing/selftests/kvm/x86/guest_memfd_conversions_test.c
> +++ b/tools/testing/selftests/kvm/x86/guest_memfd_conversions_test.c
> @@ -61,8 +61,13 @@ static void gmem_conversions_do_setup(test_data_t *t, int nr_pages,
>
>  static void gmem_conversions_do_teardown(test_data_t *t)
>  {
> +       /* Use NULL to avoid second free in FIXTURE_TEARDOWN (multipage tests). */
> +       if (!t->vcpu)
> +               return;
> +
>         /* No need to close gmem_fd, it's owned by the VM structure. */
>         kvm_vm_free(t->vcpu->vm);
> +       t->vcpu = NULL;
>  }
>
>  FIXTURE_TEARDOWN(gmem_conversions)
> @@ -101,6 +106,29 @@ static void __gmem_conversions_##test(test_data_t *t, int nr_pages)                \
>  #define GMEM_CONVERSION_TEST_INIT_SHARED(test)                                 \
>         __GMEM_CONVERSION_TEST_INIT_SHARED(test, 1)
>
> +/*
> + * Repeats test over nr_pages in a guest_memfd of size nr_pages, providing each
> + * test iteration with test_page, the index of the page under test in
> + * guest_memfd. test_page takes values 0..(nr_pages - 1) inclusive.
> + */
> +#define GMEM_CONVERSION_MULTIPAGE_TEST_INIT_SHARED(test, __nr_pages)           \
> +static void __gmem_conversions_multipage_##test(test_data_t *t, int nr_pages,  \
> +                                               const int test_page);           \
> +                                                                               \
> +TEST_F(gmem_conversions, test)                                                 \
> +{                                                                              \
> +       const u64 flags = GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_INIT_SHARED; \
> +       int i;                                                                  \
> +                                                                               \
> +       for (i = 0; i < __nr_pages; ++i) {                                      \
> +               gmem_conversions_do_setup(self, __nr_pages, flags);             \
> +               __gmem_conversions_multipage_##test(self, __nr_pages, i);       \
> +               gmem_conversions_do_teardown(self);                             \
> +       }                                                                       \
> +}                                                                              \
> +static void __gmem_conversions_multipage_##test(test_data_t *t, int nr_pages,  \
> +                                               const int test_page)
> +
>  struct guest_check_data {
>         void *mem;
>         char expected_val;
> @@ -199,6 +227,44 @@ GMEM_CONVERSION_TEST_INIT_SHARED(init_shared)
>         test_convert_to_shared(t, 0, 'C', 'D', 'E');
>  }
>
> +GMEM_CONVERSION_MULTIPAGE_TEST_INIT_SHARED(indexing, 4)
> +{
> +       int i;
> +
> +       /* Get a char that varies with both i and n. */
> +#define combine(x, n) ((x << 4) + (n))
> +#define i_(n) (combine(i, n))
> +#define t_(n) (combine(test_page, n))
> +
> +       /*
> +        * Start with the highest index, to catch any errors when, perhaps, the
> +        * first page is returned even for the last index.
> +        */
> +       for (i = nr_pages - 1; i >= 0; --i)
> +               test_shared(t, i, 0, i_(0), i_(2));
> +
> +       test_convert_to_private(t, test_page, t_(2), t_(3));
> +
> +       for (i = 0; i < nr_pages; ++i) {
> +               if (i == test_page)
> +                       test_private(t, test_page, t_(3), t_(4));
> +               else
> +                       test_shared(t, i, i_(2), i_(3), i_(4));
> +       }
> +
> +       test_convert_to_shared(t, test_page, t_(4), t_(5), t_(6));
> +
> +       for (i = 0; i < nr_pages; ++i) {
> +               char expected = i == test_page ? t_(6) : i_(4);
> +
> +               test_shared(t, i, expected, i_(7), i_(8));
> +       }
> +
> +#undef t_
> +#undef i_
> +#undef combine
> +}
> +
>  int main(int argc, char *argv[])
>  {
>         TEST_REQUIRE(kvm_check_cap(KVM_CAP_VM_TYPES) & BIT(KVM_X86_SW_PROTECTED_VM));
>
> --
> 2.55.0.rc0.738.g0c8ab3ebcc-goog
>
>