Re: [PATCH v3 1/3] selftests/resctrl: Skip L3_CAT when no exclusive cache portion exists

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Richard,

On 5/28/26 7:23 PM, Richard Cheng wrote:
> L3_CAT measures cache isolation, which requires at least one cache bit
> that is not shared with non-CPU agents, i.e. cbm_mask & ~shareable_bits
> must be non-zero. On MPAM, shareable_bits == cbm_mask is a legitimate
> state, so there are situations in which no bit can be reported as
> exclusive.

To make problem clear, please append "... causing the L3_CAT test to fail."

> 
> Previously get_mask_no_shareable() was invoked inside cat_run_test()

Written in past tense is confusing since it describes current behavior.

> and silently returned -1, which surfaced as a test failure on arm64
> MPAM systems.
> 
> Implement cat_feature_check() to perform the same check at feature-check
> time. It prints a diagnostic and returns false so the test case is
> skipped instead of failing.

The second and third paragraph describes a lot of detail that is clear from
the patch. I think these two paragraphs can just be replaced with something
brief like:
	Skip, don't fail, the L3_CAT test when the platform does not have any
	cache portions that the test can use.
	

> 
> Tested-by: Chen Yu <yu.c.chen@intel.com>
> Signed-off-by: Richard Cheng <icheng@nvidia.com>
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> ---
>  tools/testing/selftests/resctrl/cat_test.c | 23 +++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> index f00b622c1460..dc414e55ae94 100644
> --- a/tools/testing/selftests/resctrl/cat_test.c
> +++ b/tools/testing/selftests/resctrl/cat_test.c
> @@ -378,11 +378,32 @@ static bool noncont_cat_feature_check(const struct resctrl_test *test)
>  	return resource_info_file_exists(test->resource, "sparse_masks");
>  }
>  
> +static bool cat_feature_check(const struct resctrl_test *test)
> +{
> +	unsigned long mask;
> +
> +	if (!test_resource_feature_check(test))
> +		return false;
> +
> +	/*
> +	 * The CAT isolation measurement needs a cache portion that no
> +	 * other agent shares. On MPAM the kernel may legitimately report

This does not seem to be unique to MPAM (https://lore.kernel.org/lkml/c9bee1ad-10f8-4e69-8d90-609cdd72580e@intel.com/)
Perhaps "On MPAM the kernel may" -> "Some platforms may"?

> +	 * all bits as shareable; skip the test if that is the case.
> +	 */
> +	if (get_mask_no_shareable(test->resource, &mask)) {
> +		ksft_print_msg("All %s bits are shareable; cannot measure CAT isolation\n",
> +			       test->resource);
> +		return false;
> +	}
> +
> +	return true;
> +}
> +
>  struct resctrl_test l3_cat_test = {
>  	.name = "L3_CAT",
>  	.group = "CAT",
>  	.resource = "L3",
> -	.feature_check = test_resource_feature_check,
> +	.feature_check = cat_feature_check,
>  	.run_test = cat_run_test,
>  	.cleanup = cat_test_cleanup,
>  };

Patch looks good to me, thank you.

Reinette