Re: [PATCH v4 1/3] selftests/resctrl: Skip L3_CAT when no exclusive cache portion exists

[Richard Cheng <icheng@nvidia.com>]

On Wed, Aug 12, 2026 at 10:22:03AM +0800, Reinette Chatre wrote:
> Hi Richard,
> 
> On 7/21/26 8:59 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.
> > 
> > Previously get_mask_no_shareable() was invoked inside cat_run_test()
> > 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.
> > 
> > Tested-by: Chen Yu <yu.c.chen@intel.com>
> > Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> > Signed-off-by: Richard Cheng <icheng@nvidia.com>
> > ---
> 
> This patch is unchanged from v3. My comments against v3 still apply:
> https://lore.kernel.org/lkml/5d76e776-82d3-4f02-8384-a890349ae96c@intel.com/
> 
> As noted there, this issue does not just impact aarch64. Could you please
> split this patch from this series? When it is ready I would like to make a
> request for its inclusion in the next cycle.
> 
> Thank you.
> 
> Reinette

Hi Reinette,

Thanks for your patient and review.

Just to confirm that I understand correctly before I send v5.

I plan to
1. Split this patch out of the arm64 series and send it as a standalone v5 patch
2. Keep the code change the same, and udpate the commit message to make it clear that having no exclusive cache portion causes
   L3_CAT to fail, and simplify the desciprtion as you suggested
3. Change the in-code comment to platform-neutral wording

About the editted commit message, I plan to write it like the following

"""
selftests/resctrl: Skip L3_CAT when no exclusive cache portion exists

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. Some platforms legitimately report every cache bit as
shareable, leaving no exclusive cache portion and causing L3_CAT to fail.

Skip, rather than fail, the L3_CAT test when the platform has no
exclusive cache portion that the test can use.
"""

Does this match what you imagined ?

Best regards,
Richard Cheng.