Re: [PATCH v2 27/29] arm_mpam: Add helper to reset saved mbwu state

Fenghua Yu
2025-09-26T04:11:53Z

Hi, James,

On 9/10/25 13:43, James Morse wrote:
> resctrl expects to reset the bandwidth counters when the filesystem
> is mounted.
>
> To allow this, add a helper that clears the saved mbwu state. Instead
> of cross calling to each CPU that can access the component MSC to
> write to the counter, set a flag that causes it to be zero'd on the
> the next read. This is easily done by forcing a configuration update.
>
> Signed-off-by: James Morse <james.morse@arm.com>

Other than the following minor change,

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

> ---
> drivers/resctrl/mpam_devices.c | 47 +++++++++++++++++++++++++++++++--
> drivers/resctrl/mpam_internal.h | 5 +++-
> 2 files changed, 49 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 3080a81f0845..8254d6190ca2 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -1088,9 +1088,11 @@ static u64 mpam_msmon_overflow_val(struct mpam_msc_ris *ris)
> static void __ris_msmon_read(void *arg)
> {
> bool nrdy = false;
> + bool config_mismatch;
> struct mon_read *m = arg;
> u64 now, overflow_val = 0;
> struct mon_cfg *ctx = m->ctx;
> + bool reset_on_next_read = false;
> struct mpam_msc_ris *ris = m->ris;
> struct msmon_mbwu_state *mbwu_state;
> struct mpam_props *rprops = &ris->props;
> @@ -1105,6 +1107,14 @@ static void __ris_msmon_read(void *arg)
> FIELD_PREP(MSMON_CFG_MON_SEL_RIS, ris->ris_idx);
> mpam_write_monsel_reg(msc, CFG_MON_SEL, mon_sel);
>
> + if (m->type == mpam_feat_msmon_mbwu) {
> + mbwu_state = &ris->mbwu_state[ctx->mon];
> + if (mbwu_state) {
> + reset_on_next_read = mbwu_state->reset_on_next_read;
> + mbwu_state->reset_on_next_read = false;
> + }
> + }
> +
> /*
> * Read the existing configuration to avoid re-writing the same values.
> * This saves waiting for 'nrdy' on subsequent reads.
> @@ -1112,7 +1122,10 @@ static void __ris_msmon_read(void *arg)
> read_msmon_ctl_flt_vals(m, &cur_ctl, &cur_flt);
> clean_msmon_ctl_val(&cur_ctl);
> gen_msmon_ctl_flt_vals(m, &ctl_val, &flt_val);
> - if (cur_flt != flt_val || cur_ctl != (ctl_val | MSMON_CFG_x_CTL_EN))
> + config_mismatch = cur_flt != flt_val ||
> + cur_ctl != (ctl_val | MSMON_CFG_x_CTL_EN);
> +
> + if (config_mismatch || reset_on_next_read)
> write_msmon_ctl_flt_vals(m, ctl_val, flt_val);
>
> switch (m->type) {
> @@ -1145,7 +1158,6 @@ static void __ris_msmon_read(void *arg)
> if (nrdy)
> break;
>
> - mbwu_state = &ris->mbwu_state[ctx->mon];
> if (!mbwu_state)
> break;
>
> @@ -1245,6 +1257,37 @@ int mpam_msmon_read(struct mpam_component *comp, struct mon_cfg *ctx,
> return err;
> }
>
> +void mpam_msmon_reset_mbwu(struct mpam_component *comp, struct mon_cfg *ctx)
> +{
> + int idx;
> + struct mpam_msc *msc;
> + struct mpam_vmsc *vmsc;
> + struct mpam_msc_ris *ris;
> +
> + if (!mpam_is_enabled())
> + return;
> +
> + idx = srcu_read_lock(&mpam_srcu);
guard(srcu)(&mpam_srcu);
> + list_for_each_entry_rcu(vmsc, &comp->vmsc, comp_list) {
> + if (!mpam_has_feature(mpam_feat_msmon_mbwu, &vmsc->props))
> + continue;
> +
> + msc = vmsc->msc;
> + list_for_each_entry_rcu(ris, &vmsc->ris, vmsc_list) {
> + if (!mpam_has_feature(mpam_feat_msmon_mbwu, &ris->props))
> + continue;
> +
> + if (WARN_ON_ONCE(!mpam_mon_sel_lock(msc)))
> + continue;
> +
> + ris->mbwu_state[ctx->mon].correction = 0;
> + ris->mbwu_state[ctx->mon].reset_on_next_read = true;
> + mpam_mon_sel_unlock(msc);
> + }
> + }
> + srcu_read_unlock(&mpam_srcu, idx);
> +}
> +
> static void mpam_reset_msc_bitmap(struct mpam_msc *msc, u16 reg, u16 wd)
> {
> u32 num_words, msb;
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index c190826dfbda..7cbcafe8294a 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -223,10 +223,12 @@ struct mon_cfg {
>
> /*
> * Changes to enabled and cfg are protected by the msc->lock.
> - * Changes to prev_val and correction are protected by the msc's mon_sel_lock.
> + * Changes to reset_on_next_read, prev_val and correction are protected by the
> + * msc's mon_sel_lock.
> */
> struct msmon_mbwu_state {
> bool enabled;
> + bool reset_on_next_read;
> struct mon_cfg cfg;
>
> /* The value last read from the hardware. Used to detect overflow. */
> @@ -393,6 +395,7 @@ int mpam_apply_config(struct mpam_component *comp, u16 partid,
>
> int mpam_msmon_read(struct mpam_component *comp, struct mon_cfg *ctx,
> enum mpam_device_features, u64 *val);
> +void mpam_msmon_reset_mbwu(struct mpam_component *comp, struct mon_cfg *ctx);
>
> int mpam_get_cpumask_from_cache_id(unsigned long cache_id, u32 cache_level,
> cpumask_t *affinity);

Thanks.

-Fenghua