L3_CAT measures cache isolation, which requires at least one ache bit
not shared with non-CPU agents, i.e. cbm_mask & ~shareable_bits != 0 .
On MPAM, shareable_bits == cbm_mask is a legitimate state, so there
might be situations that no bit can be reported as exclusive.

Previously get_mask_no_shareable() does the check inside cat_run_test(),
which returns -1 silently and surfaced as a failure for arm64 MPAM.

Implement cat_feature_check() to run the same check at feature-check
time, it prints a diagnostic and return false so the test case itself
can know when to skip.

Signed-off-by: Richard Cheng <icheng@nvidia.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 37ca70523ed2..6a25efa59837 100644
--- a/tools/testing/selftests/resctrl/cat_test.c
+++ b/tools/testing/selftests/resctrl/cat_test.c
@@ -383,11 +383,32 @@ 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 all bits as shareable, skip if that 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,
 };
-- 
2.43.0

Hi Richard,

On 5/26/2026 10:42 AM, Richard Cheng wrote:
> L3_CAT measures cache isolation, which requires at least one ache bit
> not shared with non-CPU agents, i.e. cbm_mask & ~shareable_bits != 0 .
> On MPAM, shareable_bits == cbm_mask is a legitimate state, so there
> might be situations that no bit can be reported as exclusive.
> 
> Previously get_mask_no_shareable() does the check inside cat_run_test(),
> which returns -1 silently and surfaced as a failure for arm64 MPAM.
> 
> Implement cat_feature_check() to run the same check at feature-check
> time, it prints a diagnostic and return false so the test case itself
> can know when to skip.
> 
> Signed-off-by: Richard Cheng <icheng@nvidia.com>

We have also observed this issue, and the patch helps identify the root 
cause
of the CAT test failure.

./resctrl_tests -t L3_CAT
TAP version 13
# Pass: Check kernel supports resctrl filesystem
# Pass: Check resctrl mountpoint "/sys/fs/resctrl" exists
# resctrl filesystem not mounted
# dmesg: [   43.705792] resctrl: L3 allocation detected
# dmesg: [   43.712878] resctrl: L2 allocation detected
# dmesg: [   43.719856] resctrl: MB allocation detected
# dmesg: [   43.726800] resctrl: L3 monitoring detected
1..1
# Starting L3_CAT test ...
# Mounting resctrl to "/sys/fs/resctrl"
# All L3 bits are shareable; cannot measure CAT isolation
ok 1 # SKIP Hardware does not support L3_CAT or L3_CAT is disabled
# 1 skipped test(s) detected. Consider enabling relevant config options 
to improve coverage.
# Totals: pass:0 fail:0 xfail:0 xpass:0 skip:1 error:0

Tested-by: Chen Yu <yu.c.chen@intel.com>

thanks,
Chenyu