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

[Shanker Donthineni]

Hi Ben,

On 7/28/2026 5:48 AM, Ben Horgan wrote:
> External email: Use caution opening links or attachments
>
>
> 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 :)
The original commit message for dc48eb1ff27c explicitly states that 
T241-MPAM-6 affects both
MSMON_MBWU and MSMON_MBWU_L:

 "The registers MSMON_MBWU_L and MSMON_MBWU return the number of 
requests rather than the number
  of bytes transferred."

LWD (when HAS_LONG=1) selects the width/format of MSMON_MBWU_L; it does 
not identify a separate
register outside the scope described above.

> > 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)
T241 does not implement RIS (MPAMF_IDR.HAS_RIS=0). Its L3 MSC implements 
a 63-bit long bandwidth
counter, as indicated by HAS_LONG=1 and LWD=1; therefore, there are no 
per-RIS differences in
counter width.

The mpam_feat_msmon_mbwu_63counter exclusion in dc48eb1ff27c means the 
workaround is not applied
to the long-counter format implemented by the T241 L3 MSC. This patch 
removes that exclusion and
makes the implementation consistent with the original erratum 
description and commit message.

> 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)