Re: [PATCH] arm_mpam: Apply T241-MPAM-6 to 63-bit counters

[Ben Horgan]

Hi Shanker,

On 7/27/26 20:13, Shanker Donthineni wrote:
> T241-MPAM-6 causes all MBWU counter formats to count 64-byte
> requests instead of bytes. Commit dc48eb1ff27c excluded the 63-bit

As described in that commit message, T241-MPAM-6 does not affect LWD counters. Is there another
canonical description of T241-MPAM-6. Otherwise, this just looks like you've discovered a new h/w
bug :)

> MSMON_MBWU_LWD format while scaling the shorter counters. Systems
> selecting the preferred 63-bit counter consequently report bandwidth
> values that are 64 times too small.

So, the MSC with IIDR MPAM_IIDR_NVIDIA_T241 has different sizes of long bandwidth counters between
RIS. This seems a bit unusual. Please could you share which RIS have 44 bit long counters and which
have 63 bits. (MPAMF_MBWUMON_IDR.LWD gives this)

Thanks,

Ben

> 
> Apply the scale to both the sampled value and overflow correction for
> the 63-bit format. Unsigned arithmetic retains modulo-u64 behavior
> when the scaled counter range exceeds u64.
> 
> Fixes: dc48eb1ff27c ("arm_mpam: Add workaround for T241-MPAM-6")
> Link: https://lore.kernel.org/lkml/20240816131432.993859-1-sdonthineni@xxxxxxxxxx/
> Signed-off-by: Shanker Donthineni <sdonthineni@xxxxxxxxxx>
> ---
>  drivers/resctrl/mpam_devices.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 2f09f4b78bd3b..b68f5599e8dbe 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -1196,8 +1196,7 @@ static u64 mpam_msmon_overflow_val(enum mpam_device_features type,
>  {
>  	u64 overflow_val = __mpam_msmon_overflow_val(type);
>  
> -	if (mpam_has_quirk(T241_MBW_COUNTER_SCALE_64, msc) &&
> -	    type != mpam_feat_msmon_mbwu_63counter)
> +	if (mpam_has_quirk(T241_MBW_COUNTER_SCALE_64, msc))
>  		overflow_val *= 64;
>  
>  	return overflow_val;
> @@ -1293,8 +1292,7 @@ static void __ris_msmon_read(void *arg)
>  			now = FIELD_GET(MSMON___VALUE, now);
>  		}
>  
> -		if (mpam_has_quirk(T241_MBW_COUNTER_SCALE_64, msc) &&
> -		    m->type != mpam_feat_msmon_mbwu_63counter)
> +		if (mpam_has_quirk(T241_MBW_COUNTER_SCALE_64, msc))
>  			now *= 64;
>  
>  		if (nrdy)