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

[Fenghua Yu]

Hi, Ben,

On 2/24/26 09:57, Ben Horgan wrote:
> From: Shanker Donthineni <sdonthineni@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Tested-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxxxxx>
> Reviewed-by: Shaopeng Tan <tan.shaopeng@xxxxxxxxxxxxxx>
> Signed-off-by: Shanker Donthineni <sdonthineni@xxxxxxxxxx>
> Signed-off-by: James Morse <james.morse@xxxxxxx>
> Signed-off-by: Ben Horgan <ben.horgan@xxxxxxx>

Reviewed-by: Fenghua Yu <fenghuay@xxxxxxxxxx>

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@xxxxxxxxxx/T/#u

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

Thanks.

-Fenghua