Re: [PATCH 2/3] selftests/resctrl: Add L3_CAT_VALIDATE to check invalid CBMs

[Reinette Chatre]

Hi Richard,

Thank you very much. This is useful.

On 6/8/26 4:06 AM, Richard Cheng wrote:
> A CAT allocation is a CBM, and the kernel must refuse masks that can't
> describe a valid allocation. Nothing in the suite check this.
> 
> Implement L3_CAT_VALIDATE, first confirm a valid full mask is accepted,

How about "L3_CBM_VALIDATE"?

> then write an empty mask and a mask with bits outside cbm_mask and
> confirm each is refused. A mask below min_cbm_bits is also checked where

The empty mask is not always a valid test (more below).

> the platform needs more than one bit.
(nit: please write changelog in imperative tone)

> 
> Signed-off-by: Richard Cheng <icheng@xxxxxxxxxx>
> ---
>  tools/testing/selftests/resctrl/cat_test.c    | 79 +++++++++++++++++++
>  tools/testing/selftests/resctrl/resctrl.h     |  1 +
>  .../testing/selftests/resctrl/resctrl_tests.c |  1 +
>  3 files changed, 81 insertions(+)
> 
> diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> index 16a947f1ed16..d236988916d9 100644
> --- a/tools/testing/selftests/resctrl/cat_test.c
> +++ b/tools/testing/selftests/resctrl/cat_test.c
> @@ -603,3 +603,82 @@ struct resctrl_test l3_cat_occup_test = {
>  	.run_test = cat_occup_run_test,
>  	.cleanup = cat_occup_test_cleanup,
>  };
> +
> +/*
> + * L3_CAT_VALIDATE - Verify the kernel rejects invalid L3 CBM writes.
> + *
> + * CAT allocation accepts a Capacity Bit Mask (CBM). The kernel must reject
> + * masks that cannot represent a valid allocation: an empty mask, a mask with
> + * bits outside cbm_mask, or (where the platform requires it) a mask with
> + * fewer than min_cbm_bits bits. A rejected schemata write makes
> + * write_schemata() return non-zero, which is the signal this test checks.
> + */
> +static int cat_validate_run_test(const struct resctrl_test *test,

All tests in this file are "cat" tests so this function name could be more
specific. How about "cbm_validate_run_test()"?

> +				 const struct user_params *uparams)
> +{
> +	unsigned long full_mask, bad_mask;
> +	unsigned int min_cbm_bits, start;
> +	char schemata[64];
> +	int count_of_bits, ret;

(reverse fir please)

> +
> +	ret = get_full_cbm(test->resource, &full_mask);
> +	if (ret)
> +		return ret;
> +
> +	ret = resource_info_unsigned_get(test->resource, "min_cbm_bits",
> +					 &min_cbm_bits);
> +	if (ret)
> +		return ret;
> +
> +	count_of_bits = count_bits(full_mask);
> +
> +	/* A valid full CBM must be accepted */
> +	snprintf(schemata, sizeof(schemata), "%lx", full_mask);
> +	if (write_schemata("", schemata, uparams->cpu, test->resource)) {
> +		ksft_print_msg("Valid CBM 0x%lx was rejected\n", full_mask);
> +		return 1;

KSFT_FAIL?

> +	}
> +
> +	/* An empty mask must be rejected. */

Some systems do support an empty mask (AMD) so this needs to depend on min_cbm_bits
being larger than zero.

> +	if (!write_schemata("", "0", uparams->cpu, test->resource)) {
> +		ksft_print_msg("Empty CBM was accepted, must be rejected\n");
> +		return 1;
> +	}
> +
> +	/* A mask with a bit outside cbm_mask must be rejected. */
> +	bad_mask = full_mask | (1UL << count_of_bits);
> +	snprintf(schemata, sizeof(schemata), "%lx", bad_mask);
> +	if (!write_schemata("", schemata, uparams->cpu, test->resource)) {
> +		ksft_print_msg("Out-of-range CBM 0x%lx was accepted, must be rejected\n",
> +			       bad_mask);
> +		return 1;

KSFT_FAIL?

> +	}
> +
> +	/*
> +	 * A mask with fewer than min_cbm_bits bits must be rejected. When
> +	 * min_cbm_bits is 1 the only sub-minimum mask is the empty mask
> +	 * This case only applies when the platform requires more contiguous bits.
> +	 */
> +	if (min_cbm_bits > 1) {
> +		count_contiguous_bits(full_mask, &start);

The return value should be checked here to ensure that the create_bit_mask() below 
is valid.

> +		bad_mask = create_bit_mask(start, min_cbm_bits - 1);
> +		snprintf(schemata, sizeof(schemata), "%lx", bad_mask);
> +		if (!write_schemata("", schemata, uparams->cpu, test->resource)) {
> +			ksft_print_msg("CBM 0x%lx with too few bits was accepted\n",
> +				       bad_mask);
> +			return 1;

KSFT_FAIL?

> +		}
> +	}
> +
> +	ksft_print_msg("Pass: CAT rejects invalid CBM writes\n");
> +
> +	return 0;
> +}
> +
> +struct resctrl_test l3_cat_validate_test = {
> +	.name = "L3_CAT_VALIDATE",
> +	.group = "CAT",
> +	.resource = "L3",
> +	.feature_check = test_resource_feature_check,
> +	.run_test = cat_validate_run_test,
> +};
> diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
> index ce3abf0bdac2..e2e3cf7833bc 100644
> --- a/tools/testing/selftests/resctrl/resctrl.h
> +++ b/tools/testing/selftests/resctrl/resctrl.h
> @@ -248,6 +248,7 @@ extern struct resctrl_test mba_test;
>  extern struct resctrl_test cmt_test;
>  extern struct resctrl_test l3_cat_test;
>  extern struct resctrl_test l3_cat_occup_test;
> +extern struct resctrl_test l3_cat_validate_test;
>  extern struct resctrl_test l3_noncont_cat_test;
>  extern struct resctrl_test l2_noncont_cat_test;
>  
> diff --git a/tools/testing/selftests/resctrl/resctrl_tests.c b/tools/testing/selftests/resctrl/resctrl_tests.c
> index 324a60818aa1..66739e96f33c 100644
> --- a/tools/testing/selftests/resctrl/resctrl_tests.c
> +++ b/tools/testing/selftests/resctrl/resctrl_tests.c
> @@ -20,6 +20,7 @@ static struct resctrl_test *resctrl_tests[] = {
>  	&cmt_test,
>  	&l3_cat_test,
>  	&l3_cat_occup_test,
> +	&l3_cat_validate_test,
>  	&l3_noncont_cat_test,
>  	&l2_noncont_cat_test,
>  };

Thank you.

Reinette