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

["Chen, Yu C" <yu.c.chen@intel.com>]

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