Re: [PATCH v2 24/29] arm_mpam: Track bandwidth counter state for overflow and power management

Ben Horgan
2025-09-12T15:55:22Z

Hi James,

On 9/10/25 21:43, James Morse wrote:
> Bandwidth counters need to run continuously to correctly reflect the
> bandwidth.
>
> The value read may be lower than the previous value read in the case
> of overflow and when the hardware is reset due to CPU hotplug.
>
> Add struct mbwu_state to track the bandwidth counter to allow overflow
> and power management to be handled.
>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Changes since v1:
> * Fixed lock/unlock typo.
> ---
> drivers/resctrl/mpam_devices.c | 154 +++++++++++++++++++++++++++++++-
> drivers/resctrl/mpam_internal.h | 23 +++++
> 2 files changed, 175 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index 1543c33c5d6a..eeb62ed94520 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -918,6 +918,7 @@ static void gen_msmon_ctl_flt_vals(struct mon_read *m, u32 *ctl_val,
> *ctl_val |= MSMON_CFG_x_CTL_MATCH_PARTID;
>
> *flt_val = FIELD_PREP(MSMON_CFG_x_FLT_PARTID, ctx->partid);
> +
> if (m->ctx->match_pmg) {
> *ctl_val |= MSMON_CFG_x_CTL_MATCH_PMG;
> *flt_val |= FIELD_PREP(MSMON_CFG_x_FLT_PMG, ctx->pmg);
> @@ -972,6 +973,7 @@ static void clean_msmon_ctl_val(u32 *cur_ctl)
> static void write_msmon_ctl_flt_vals(struct mon_read *m, u32 ctl_val,
> u32 flt_val)
> {
> + struct msmon_mbwu_state *mbwu_state;
> struct mpam_msc *msc = m->ris->vmsc->msc;
>
> /*
> @@ -990,20 +992,32 @@ static void write_msmon_ctl_flt_vals(struct mon_read *m, u32 ctl_val,
> mpam_write_monsel_reg(msc, CFG_MBWU_CTL, ctl_val);
> mpam_write_monsel_reg(msc, MBWU, 0);
> mpam_write_monsel_reg(msc, CFG_MBWU_CTL, ctl_val | MSMON_CFG_x_CTL_EN);
> +
> + mbwu_state = &m->ris->mbwu_state[m->ctx->mon];
> + if (mbwu_state)
> + mbwu_state->prev_val = 0;

What's the if condition doing here?

The below could make more sense but I don't think you can get here if
the allocation fails.

if (m->ris->mbwu_state)

> +
> break;
> default:
> return;
> }
> }
>
> +static u64 mpam_msmon_overflow_val(struct mpam_msc_ris *ris)
> +{
> + /* TODO: scaling, and long counters */
> + return GENMASK_ULL(30, 0);
> +}
> +
> /* Call with MSC lock held */
> static void __ris_msmon_read(void *arg)
> {
> - u64 now;
> bool nrdy = false;
> struct mon_read *m = arg;
> + u64 now, overflow_val = 0;
> struct mon_cfg *ctx = m->ctx;
> struct mpam_msc_ris *ris = m->ris;
> + struct msmon_mbwu_state *mbwu_state;
> struct mpam_props *rprops = &ris->props;
> struct mpam_msc *msc = m->ris->vmsc->msc;
> u32 mon_sel, ctl_val, flt_val, cur_ctl, cur_flt;
> @@ -1031,11 +1045,30 @@ static void __ris_msmon_read(void *arg)
> now = mpam_read_monsel_reg(msc, CSU);
> if (mpam_has_feature(mpam_feat_msmon_csu_hw_nrdy, rprops))
> nrdy = now & MSMON___NRDY;
> + now = FIELD_GET(MSMON___VALUE, now);
> break;
> case mpam_feat_msmon_mbwu:
> now = mpam_read_monsel_reg(msc, MBWU);
> if (mpam_has_feature(mpam_feat_msmon_mbwu_hw_nrdy, rprops))
> nrdy = now & MSMON___NRDY;
> + now = FIELD_GET(MSMON___VALUE, now);
> +
> + if (nrdy)
> + break;
> +
> + mbwu_state = &ris->mbwu_state[ctx->mon];
> + if (!mbwu_state)
> + break;
> +
> + /* Add any pre-overflow value to the mbwu_state->val */
> + if (mbwu_state->prev_val > now)
> + overflow_val = mpam_msmon_overflow_val(ris) - mbwu_state->prev_val;
> +
> + mbwu_state->prev_val = now;
> + mbwu_state->correction += overflow_val;
> +
> + /* Include bandwidth consumed before the last hardware reset */
> + now += mbwu_state->correction;
> break;
> default:
> m->err = -EINVAL;
> @@ -1048,7 +1081,6 @@ static void __ris_msmon_read(void *arg)
> return;
> }
>
> - now = FIELD_GET(MSMON___VALUE, now);
> *m->val += now;
> }
>
> @@ -1261,6 +1293,67 @@ static int mpam_reprogram_ris(void *_arg)
> return 0;
> }
>
> +/* Call with MSC lock held */
> +static int mpam_restore_mbwu_state(void *_ris)
> +{
> + int i;
> + struct mon_read mwbu_arg;
> + struct mpam_msc_ris *ris = _ris;
> +
> + for (i = 0; i < ris->props.num_mbwu_mon; i++) {
> + if (ris->mbwu_state[i].enabled) {
> + mwbu_arg.ris = ris;
> + mwbu_arg.ctx = &ris->mbwu_state[i].cfg;
> + mwbu_arg.type = mpam_feat_msmon_mbwu;
> +
> + __ris_msmon_read(&mwbu_arg);
> + }
> + }
> +
> + return 0;
> +}
> +
> +/* Call with MSC lock and held */
> +static int mpam_save_mbwu_state(void *arg)
> +{
> + int i;
> + u64 val;
> + struct mon_cfg *cfg;
> + u32 cur_flt, cur_ctl, mon_sel;
> + struct mpam_msc_ris *ris = arg;
> + struct msmon_mbwu_state *mbwu_state;
> + struct mpam_msc *msc = ris->vmsc->msc;
> +
> + for (i = 0; i < ris->props.num_mbwu_mon; i++) {
> + mbwu_state = &ris->mbwu_state[i];
> + cfg = &mbwu_state->cfg;
> +
> + if (WARN_ON_ONCE(!mpam_mon_sel_lock(msc)))
> + return -EIO;
> +
> + mon_sel = FIELD_PREP(MSMON_CFG_MON_SEL_MON_SEL, i) |
> + FIELD_PREP(MSMON_CFG_MON_SEL_RIS, ris->ris_idx);
> + mpam_write_monsel_reg(msc, CFG_MON_SEL, mon_sel);
> +
> + cur_flt = mpam_read_monsel_reg(msc, CFG_MBWU_FLT);
> + cur_ctl = mpam_read_monsel_reg(msc, CFG_MBWU_CTL);
> + mpam_write_monsel_reg(msc, CFG_MBWU_CTL, 0);
> +
> + val = mpam_read_monsel_reg(msc, MBWU);
> + mpam_write_monsel_reg(msc, MBWU, 0);
> +
> + cfg->mon = i;
> + cfg->pmg = FIELD_GET(MSMON_CFG_x_FLT_PMG, cur_flt);
> + cfg->match_pmg = FIELD_GET(MSMON_CFG_x_CTL_MATCH_PMG, cur_ctl);
> + cfg->partid = FIELD_GET(MSMON_CFG_x_FLT_PARTID, cur_flt);
> + mbwu_state->correction += val;
> + mbwu_state->enabled = FIELD_GET(MSMON_CFG_x_CTL_EN, cur_ctl);
> + mpam_mon_sel_unlock(msc);
> + }
> +
> + return 0;
> +}
> +
> static void mpam_init_reset_cfg(struct mpam_config *reset_cfg)
> {
> memset(reset_cfg, 0, sizeof(*reset_cfg));
> @@ -1335,6 +1428,9 @@ static void mpam_reset_msc(struct mpam_msc *msc, bool online)
> * for non-zero partid may be lost while the CPUs are offline.
> */
> ris->in_reset_state = online;
> +
> + if (mpam_is_enabled() && !online)
> + mpam_touch_msc(msc, &mpam_save_mbwu_state, ris);
> }
> }
>
> @@ -1369,6 +1465,9 @@ static void mpam_reprogram_msc(struct mpam_msc *msc)
> mpam_reprogram_ris_partid(ris, partid, cfg);
> }
> ris->in_reset_state = reset;
> +
> + if (mpam_has_feature(mpam_feat_msmon_mbwu, &ris->props))
> + mpam_touch_msc(msc, &mpam_restore_mbwu_state, ris);
> }
> }
>
> @@ -2091,11 +2190,33 @@ static void mpam_unregister_irqs(void)
>
> static void __destroy_component_cfg(struct mpam_component *comp)
> {
> + struct mpam_msc *msc;
> + struct mpam_vmsc *vmsc;
> + struct mpam_msc_ris *ris;
> +
> + lockdep_assert_held(&mpam_list_lock);
> +
> add_to_garbage(comp->cfg);
> + list_for_each_entry(vmsc, &comp->vmsc, comp_list) {
> + msc = vmsc->msc;
> +
> + if (mpam_mon_sel_lock(msc)) {
> + list_for_each_entry(ris, &vmsc->ris, vmsc_list)
> + add_to_garbage(ris->mbwu_state);
> + mpam_mon_sel_unlock(msc);
> + }
> + }
> }
>
> static int __allocate_component_cfg(struct mpam_component *comp)
> {
> + int err = 0;
> + struct mpam_msc *msc;
> + struct mpam_vmsc *vmsc;
> + struct mpam_msc_ris *ris;
> + struct msmon_mbwu_state *mbwu_state;
> +
> + lockdep_assert_held(&mpam_list_lock);
> mpam_assert_partid_sizes_fixed();
>
> if (comp->cfg)
> @@ -2106,6 +2227,35 @@ static int __allocate_component_cfg(struct mpam_component *comp)
> return -ENOMEM;
> init_garbage(comp->cfg);
>
> + list_for_each_entry(vmsc, &comp->vmsc, comp_list) {
> + if (!vmsc->props.num_mbwu_mon)
> + continue;
> +
> + msc = vmsc->msc;
> + list_for_each_entry(ris, &vmsc->ris, vmsc_list) {
> + if (!ris->props.num_mbwu_mon)
> + continue;
> +
> + mbwu_state = kcalloc(ris->props.num_mbwu_mon,
> + sizeof(*ris->mbwu_state),
> + GFP_KERNEL);
> + if (!mbwu_state) {
> + __destroy_component_cfg(comp);
> + err = -ENOMEM;
> + break;
> + }
> +
> + if (mpam_mon_sel_lock(msc)) {
> + init_garbage(mbwu_state);
> + ris->mbwu_state = mbwu_state;
> + mpam_mon_sel_unlock(msc);
> + }

The if statement is confusing now that mpam_mon_sel_lock()
unconditionally returns true.

> + }
> +
> + if (err)
> + break;
> + }
> +
> return 0;
> }
>
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index bb01e7dbde40..725c2aefa8a2 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -212,6 +212,26 @@ struct mon_cfg {
> enum mon_filter_options opts;
> };
>
> +/*
> + * 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.
> + */
> +struct msmon_mbwu_state {
> + bool enabled;
> + struct mon_cfg cfg;
> +
> + /* The value last read from the hardware. Used to detect overflow. */
> + u64 prev_val;
> +
> + /*
> + * The value to add to the new reading to account for power management,
> + * and shifts to trigger the overflow interrupt.
> + */
> + u64 correction;
> +
> + struct mpam_garbage garbage;
> +};
> +
> struct mpam_class {
> /* mpam_components in this class */
> struct list_head components;
> @@ -304,6 +324,9 @@ struct mpam_msc_ris {
> /* parent: */
> struct mpam_vmsc *vmsc;
>
> + /* msmon mbwu configuration is preserved over reset */
> + struct msmon_mbwu_state *mbwu_state;
> +
> struct mpam_garbage garbage;
> };
>

Thanks,

Ben