Re: [PATCH v5 38/41] arm_mpam: Add workaround for T241-MPAM-4

Fenghua Yu
2026-03-01T17:28:10Z

Hi, Ben,

On 2/24/26 09:57, Ben Horgan wrote:
> From: Shanker Donthineni <sdonthineni@nvidia.com>
>
> In the T241 implementation of memory-bandwidth partitioning, in the absence
> of contention for bandwidth, the minimum bandwidth setting can affect the
> amount of achieved bandwidth. Specifically, the achieved bandwidth in the
> absence of contention can settle to any value between the values of
> MPAMCFG_MBW_MIN and MPAMCFG_MBW_MAX. Also, if MPAMCFG_MBW_MIN is set
> zero (below 0.78125%), once a core enters a throttled state, it will never
> leave that state.
>
> The first issue is not a concern if the MPAM software allows to program
> MPAMCFG_MBW_MIN through the sysfs interface. This patch ensures program
> MBW_MIN=1 (0.78125%) whenever MPAMCFG_MBW_MIN=0 is programmed.
>
> In the scenario where the resctrl doesn't support the MBW_MIN interface via
> sysfs, to achieve bandwidth closer to MBW_MAX in the absence of contention,
> software should configure a relatively narrow gap between MBW_MIN and
> MBW_MAX. The recommendation is to use a 5% gap to mitigate the problem.
>
> Clear the feature MBW_MIN feature from the class to ensure we don't
> accidentally change behaviour when resctrl adds support for a MBW_MIN
> interface.
>
> Tested-by: Gavin Shan <gshan@redhat.com>
> Tested-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
> Signed-off-by: Shanker Donthineni <sdonthineni@nvidia.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> Signed-off-by: Ben Horgan <ben.horgan@arm.com>

Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>

This patch itself is good.

Please check the following comments.

> ---
> [ morse: Added as second quirk, adapted to use the new intermediate values
> in mpam_extend_config() ]
>
> Changes since rfc:
> MPAM_IIDR_NVIDIA_T421 -> MPAM_IIDR_NVIDIA_T241
> Handling when reset_mbw_min is set
>
> Changes since v3:
> Move the 5% gap policy back here
> Clear mbw_min feature in class
> ---
> Documentation/arch/arm64/silicon-errata.rst | 2 +
> drivers/resctrl/mpam_devices.c | 50 +++++++++++++++++++--
> drivers/resctrl/mpam_internal.h | 1 +
> 3 files changed, 50 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/arch/arm64/silicon-errata.rst b/Documentation/arch/arm64/silicon-errata.rst
> index a65620f98e3a..a4b246655e37 100644
> --- a/Documentation/arch/arm64/silicon-errata.rst
> +++ b/Documentation/arch/arm64/silicon-errata.rst
> @@ -249,6 +249,8 @@ stable kernels.
> +----------------+-----------------+-----------------+-----------------------------+
> | NVIDIA | T241 MPAM | T241-MPAM-1 | N/A |
> +----------------+-----------------+-----------------+-----------------------------+
> +| NVIDIA | T241 MPAM | T241-MPAM-4 | N/A |
> ++----------------+-----------------+-----------------+-----------------------------+
> +----------------+-----------------+-----------------+-----------------------------+
> | Freescale/NXP | LS2080A/LS1043A | A-008585 | FSL_ERRATUM_A008585 |
> +----------------+-----------------+-----------------+-----------------------------+
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 08cb080592d9..8f44e9dee207 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -679,6 +679,12 @@ static const struct mpam_quirk mpam_quirks[] = {
> .iidr_mask = MPAM_IIDR_MATCH_ONE,
> .workaround = T241_SCRUB_SHADOW_REGS,
> },
> + {
> + /* NVIDIA t241 erratum T241-MPAM-4 */
> + .iidr = MPAM_IIDR_NVIDIA_T241,
> + .iidr_mask = MPAM_IIDR_MATCH_ONE,
> + .workaround = T241_FORCE_MBW_MIN_TO_ONE,
> + },
> { NULL } /* Sentinel */
> };
>
> @@ -1464,6 +1470,31 @@ static void mpam_quirk_post_config_change(struct mpam_msc_ris *ris, u16 partid,
> mpam_apply_t241_erratum(ris, partid);
> }
>
> +static u16 mpam_wa_t241_force_mbw_min_to_one(struct mpam_props *props)
> +{
> + u16 max_hw_value, min_hw_granule, res0_bits;
> +
> + res0_bits = 16 - props->bwa_wd;
> + max_hw_value = ((1 << props->bwa_wd) - 1) << res0_bits;
> + min_hw_granule = ~max_hw_value;
> +
> + return min_hw_granule + 1;
> +}
> +
> +static u16 mpam_wa_t241_calc_min_from_max(struct mpam_config *cfg)
> +{
> + u16 val = 0;
> +
> + if (mpam_has_feature(mpam_feat_mbw_max, cfg)) {

But the problem is mpam_feat_mbw_max feature is NOT set in cfg.

> + u16 delta = ((5 * MPAMCFG_MBW_MAX_MAX) / 100) - 1;
> +
> + if (cfg->mbw_max > delta)
> + val = cfg->mbw_max - delta;
> + }
> +
> + return val;

So 0 is always returned.

The workaround will set mbw_min as 1% which is too small and will cause
performance degradation, e.g. about 20% degradation on some benchmarks.

This patch itself doesn't have any issue.

The issue is the mbw_max feature bit in cfg is not set.
This is a legacy issue, not introduced by this patch set.

Here is a fix patch for the issue:
https://lore.kernel.org/lkml/20260301171829.1357886-1-fenghuay@nvidia.com/T/#u

If the fix patch is good, could you please add it into the next version
of this series?

Thanks.

-Fenghua