Re: [PATCH v4 04/10] KVM: selftests: Read binary stat data in lib

[David Matlack]

On Mon, Apr 11, 2022 at 2:10 PM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
>
> Move the code to read the binary stats data to the KVM selftests
> library. It will be re-used by other tests to check KVM behavior.
>
> No functional change intended.
>
> Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
> ---
>  .../selftests/kvm/include/kvm_util_base.h     |  3 +++
>  .../selftests/kvm/kvm_binary_stats_test.c     | 20 +++++-------------
>  tools/testing/selftests/kvm/lib/kvm_util.c    | 21 +++++++++++++++++++
>  3 files changed, 29 insertions(+), 15 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/include/kvm_util_base.h b/tools/testing/selftests/kvm/include/kvm_util_base.h
> index c5f34551ff76..b2684cfc2cb1 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util_base.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util_base.h
> @@ -405,6 +405,9 @@ struct kvm_stats_desc *alloc_vm_stats_desc(int stats_fd,
>                                           struct kvm_stats_header *header);
>  void read_vm_stats_desc(int stats_fd, struct kvm_stats_header *header,
>                         struct kvm_stats_desc *stats_desc);
> +int read_stat_data(int stats_fd, struct kvm_stats_header *header,
> +                  struct kvm_stats_desc *desc, uint64_t *data,
> +                  ssize_t max_elements);
>
>  uint32_t guest_get_vcpuid(void);
>
> diff --git a/tools/testing/selftests/kvm/kvm_binary_stats_test.c b/tools/testing/selftests/kvm/kvm_binary_stats_test.c
> index e4795bad7db6..97b180249ba0 100644
> --- a/tools/testing/selftests/kvm/kvm_binary_stats_test.c
> +++ b/tools/testing/selftests/kvm/kvm_binary_stats_test.c
> @@ -20,6 +20,8 @@
>  #include "asm/kvm.h"
>  #include "linux/kvm.h"
>
> +#define STAT_MAX_ELEMENTS 1000
> +
>  static void stats_test(int stats_fd)
>  {
>         ssize_t ret;
> @@ -29,7 +31,7 @@ static void stats_test(int stats_fd)
>         struct kvm_stats_header header;
>         char *id;
>         struct kvm_stats_desc *stats_desc;
> -       u64 *stats_data;
> +       u64 stats_data[STAT_MAX_ELEMENTS];

What is the benefit of changing stats_data to a stack allocation with
a fixed limit?

>         struct kvm_stats_desc *pdesc;
>
>         /* Read kvm stats header */
> @@ -130,25 +132,13 @@ static void stats_test(int stats_fd)
>                         pdesc->offset, pdesc->name);
>         }
>
> -       /* Allocate memory for stats data */
> -       stats_data = malloc(size_data);
> -       TEST_ASSERT(stats_data, "Allocate memory for stats data");
> -       /* Read kvm stats data as a bulk */
> -       ret = pread(stats_fd, stats_data, size_data, header.data_offset);
> -       TEST_ASSERT(ret == size_data, "Read KVM stats data");
>         /* Read kvm stats data one by one */
> -       size_data = 0;
>         for (i = 0; i < header.num_desc; ++i) {
>                 pdesc = (void *)stats_desc + i * size_desc;
> -               ret = pread(stats_fd, stats_data,
> -                               pdesc->size * sizeof(*stats_data),
> -                               header.data_offset + size_data);
> -               TEST_ASSERT(ret == pdesc->size * sizeof(*stats_data),
> -                               "Read data of KVM stats: %s", pdesc->name);
> -               size_data += pdesc->size * sizeof(*stats_data);
> +               read_stat_data(stats_fd, &header, pdesc, stats_data,
> +                              ARRAY_SIZE(stats_data));
>         }
>
> -       free(stats_data);
>         free(stats_desc);
>         free(id);
>  }
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index e3ae26fbef03..64e2085f1129 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -2593,3 +2593,24 @@ void read_vm_stats_desc(int stats_fd, struct kvm_stats_header *header,
>         TEST_ASSERT(ret == stats_descs_size(header),
>                     "Read KVM stats descriptors");
>  }
> +
> +int read_stat_data(int stats_fd, struct kvm_stats_header *header,

I would like to keep up the practice of adding docstrings to functions
in kvm_util. Can you add docstring comments for this function and the
other kvm_util functions introduced by this series?

> +                  struct kvm_stats_desc *desc, uint64_t *data,
> +                  ssize_t max_elements)
> +{
> +       ssize_t ret;
> +
> +       TEST_ASSERT(desc->size <= max_elements,
> +                   "Max data elements should be at least as large as stat data");

What is the reason for this assertion? Callers are required to read
all the data elements of a given stat?

> +
> +       ret = pread(stats_fd, data, desc->size * sizeof(*data),
> +                   header->data_offset + desc->offset);
> +
> +       /* ret from pread is in bytes. */
> +       ret = ret / sizeof(*data);
> +
> +       TEST_ASSERT(ret == desc->size,
> +                   "Read data of KVM stats: %s", desc->name);

Won't this assertion fail when called from kvm_binary_stats_test.c?
kvm_binary_stats_test.c looks like it reads all the stat data at once,
which means ret will be the total number of stat data points, and
desc->size will be the number of stat data points in the first stat.

> +
> +       return ret;
> +}
> --
> 2.35.1.1178.g4f1659d476-goog
>