Re: [PATCH mpam mpam/snapshot/v6.14-rc1] arm64/mpam: Fix MBWU monitor overflow handling

[Zeng Heng]

Hi Ben,

On 2025/10/23 0:17, Ben Horgan wrote:
> > Also fix the handling of overflow amount calculation. There's no need to
> > subtract mbwu_state->prev_val when calculating overflow_val.
>
> Why not? Isn't this the pre-overflow part that we are missing from the
> running count?

The MSMON_MBWU register accumulates counts monotonically forward and
would not automatically cleared to zero on overflow.

The overflow portion is exactly what mpam_msmon_overflow_val() computes,
there is no need to additionally subtract mbwu_state->prev_val.

> > Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>
> > ---
> >   drivers/resctrl/mpam_devices.c | 8 +++++---
> >   1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> > index 0dd048279e02..06f3ec9887d2 100644
> > --- a/drivers/resctrl/mpam_devices.c
> > +++ b/drivers/resctrl/mpam_devices.c
> > @@ -1101,7 +1101,8 @@ static void __ris_msmon_read(void *arg)
> >   	clean_msmon_ctl_val(&cur_ctl);
> >   	gen_msmon_ctl_flt_vals(m, &ctl_val, &flt_val);
> >   	config_mismatch = cur_flt != flt_val ||
> > -			  cur_ctl != (ctl_val | MSMON_CFG_x_CTL_EN);
> > +			 (cur_ctl & ~MSMON_CFG_x_CTL_OFLOW_STATUS) !=
> > +			 (ctl_val | MSMON_CFG_x_CTL_EN);
>
> This only considers 31 bit counters. I would expect any change here to
> consider all lengths of counter. Also, as the overflow bit is no longer
> reset due to the config mismatch it needs to be reset somewhere else.

Yes, overflow bit needs to be cleared somewhere. I try to point out in
the next patch mail.

Best Regards,
Zeng Heng