Re: [PATCH v2 15/29] arm_mpam: Reset MSC controls from cpu hp callbacks

Ben Horgan
2025-09-12T11:25:33Z

Hi James,

On 9/10/25 21:42, James Morse wrote:
> When a CPU comes online, it may bring a newly accessible MSC with
> it. Only the default partid has its value reset by hardware, and
> even then the MSC might not have been reset since its config was
> previously dirtyied. e.g. Kexec.
>
> Any in-use partid must have its configuration restored, or reset.
> In-use partids may be held in caches and evicted later.
>
> MSC are also reset when CPUs are taken offline to cover cases where
> firmware doesn't reset the MSC over reboot using UEFI, or kexec
> where there is no firmware involvement.
>
> If the configuration for a RIS has not been touched since it was
> brought online, it does not need resetting again.
>
> To reset, write the maximum values for all discovered controls.
>
> CC: Rohit Mathew <Rohit.Mathew@arm.com>
> Signed-off-by: James Morse <james.morse@arm.com>
> ---
> Changes since RFC:
> * Last bitmap write will always be non-zero.
> * Dropped READ_ONCE() - teh value can no longer change.
> * Write 0 to proporitional stride, remove the bwa_fract variable.
> * Removed nested srcu lock, the assert should cover it.
> ---
> drivers/resctrl/mpam_devices.c | 117 ++++++++++++++++++++++++++++++++
> drivers/resctrl/mpam_internal.h | 8 +++
> 2 files changed, 125 insertions(+)
>
> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> index cd8e95fa5fd6..0353313cf284 100644
> --- a/drivers/resctrl/mpam_devices.c
> +++ b/drivers/resctrl/mpam_devices.c
> @@ -7,6 +7,7 @@
> #include <linux/atomic.h>
> #include <linux/arm_mpam.h>
> #include <linux/bitfield.h>
> +#include <linux/bitmap.h>
> #include <linux/cacheinfo.h>
> #include <linux/cpu.h>
> #include <linux/cpumask.h>
> @@ -777,8 +778,110 @@ static int mpam_msc_hw_probe(struct mpam_msc *msc)
> return 0;
> }
>
> +static void mpam_reset_msc_bitmap(struct mpam_msc *msc, u16 reg, u16 wd)
> +{
> + u32 num_words, msb;
> + u32 bm = ~0;
> + int i;
> +
> + lockdep_assert_held(&msc->part_sel_lock);
> +
> + if (wd == 0)
> + return;
> +
> + /*
> + * Write all ~0 to all but the last 32bit-word, which may
> + * have fewer bits...
> + */
> + num_words = DIV_ROUND_UP(wd, 32);
> + for (i = 0; i < num_words - 1; i++, reg += sizeof(bm))
> + __mpam_write_reg(msc, reg, bm);
> +
> + /*
> + * ....and then the last (maybe) partial 32bit word. When wd is a
> + * multiple of 32, msb should be 31 to write a full 32bit word.
> + */
> + msb = (wd - 1) % 32;
> + bm = GENMASK(msb, 0);
> + __mpam_write_reg(msc, reg, bm);
> +}
> +
> +static void mpam_reset_ris_partid(struct mpam_msc_ris *ris, u16 partid)
> +{
> + struct mpam_msc *msc = ris->vmsc->msc;
> + struct mpam_props *rprops = &ris->props;
> +
> + mpam_assert_srcu_read_lock_held();
> +
> + mutex_lock(&msc->part_sel_lock);
> + __mpam_part_sel(ris->ris_idx, partid, msc);
> +
> + if (mpam_has_feature(mpam_feat_cpor_part, rprops))
> + mpam_reset_msc_bitmap(msc, MPAMCFG_CPBM, rprops->cpbm_wd);
> +
> + if (mpam_has_feature(mpam_feat_mbw_part, rprops))
> + mpam_reset_msc_bitmap(msc, MPAMCFG_MBW_PBM, rprops->mbw_pbm_bits);
> +
> + if (mpam_has_feature(mpam_feat_mbw_min, rprops))
> + mpam_write_partsel_reg(msc, MBW_MIN, 0);
> +
> + if (mpam_has_feature(mpam_feat_mbw_max, rprops))
> + mpam_write_partsel_reg(msc, MBW_MAX, MPAMCFG_MBW_MAX_MAX);
> +
> + if (mpam_has_feature(mpam_feat_mbw_prop, rprops))
> + mpam_write_partsel_reg(msc, MBW_PROP, 0);

If mpam_feat_ccap_part is already in enum mpam_device_features then the
reset would belong here but I expect it is better just to introduce
mpam_feat_ccap_part later (patch 21). I also commented on this feature
introduction split on patch 13.

> + mutex_unlock(&msc->part_sel_lock);
> +}
> +
> +static void mpam_reset_ris(struct mpam_msc_ris *ris)
> +{
> + u16 partid, partid_max;
> +
> + mpam_assert_srcu_read_lock_held();
> +
> + if (ris->in_reset_state)
> + return;
> +
> + spin_lock(&partid_max_lock);
> + partid_max = mpam_partid_max;
> + spin_unlock(&partid_max_lock);
> + for (partid = 0; partid < partid_max; partid++)
> + mpam_reset_ris_partid(ris, partid);
> +}
> +
> +static void mpam_reset_msc(struct mpam_msc *msc, bool online)
> +{
> + struct mpam_msc_ris *ris;
> +
> + mpam_assert_srcu_read_lock_held();

Unneeded? Checked in list_for_each_entry_srcu().> +
> + list_for_each_entry_srcu(ris, &msc->ris, msc_list, srcu_read_lock_held(&mpam_srcu)) {
> + mpam_reset_ris(ris);
> +
> + /*
> + * Set in_reset_state when coming online. The reset state
> + * for non-zero partid may be lost while the CPUs are offline.
> + */
> + ris->in_reset_state = online;
> + }
> +}
> +
> static int mpam_cpu_online(unsigned int cpu)
> {
> + int idx;
> + struct mpam_msc *msc;
> +
> + idx = srcu_read_lock(&mpam_srcu);
> + list_for_each_entry_srcu(msc, &mpam_all_msc, all_msc_list,
> + srcu_read_lock_held(&mpam_srcu)) {
> + if (!cpumask_test_cpu(cpu, &msc->accessibility))
> + continue;
> +
> + if (atomic_fetch_inc(&msc->online_refs) == 0)
> + mpam_reset_msc(msc, true);
> + }
> + srcu_read_unlock(&mpam_srcu, idx);
> +
> return 0;
> }
>
> @@ -818,6 +921,20 @@ static int mpam_discovery_cpu_online(unsigned int cpu)
>
> static int mpam_cpu_offline(unsigned int cpu)
> {
> + int idx;
> + struct mpam_msc *msc;
> +
> + idx = srcu_read_lock(&mpam_srcu);
> + list_for_each_entry_srcu(msc, &mpam_all_msc, all_msc_list,
> + srcu_read_lock_held(&mpam_srcu)) {
> + if (!cpumask_test_cpu(cpu, &msc->accessibility))
> + continue;
> +
> + if (atomic_dec_and_test(&msc->online_refs))
> + mpam_reset_msc(msc, false);
> + }
> + srcu_read_unlock(&mpam_srcu, idx);
> +
> return 0;
> }
>
> diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
> index eace5ba871f3..6e047fbd3512 100644
> --- a/drivers/resctrl/mpam_internal.h
> +++ b/drivers/resctrl/mpam_internal.h
> @@ -5,6 +5,7 @@
> #define MPAM_INTERNAL_H
>
> #include <linux/arm_mpam.h>
> +#include <linux/atomic.h>
> #include <linux/cpumask.h>
> #include <linux/io.h>
> #include <linux/llist.h>
> @@ -45,6 +46,7 @@ struct mpam_msc {
> struct pcc_mbox_chan *pcc_chan;
> u32 nrdy_usec;
> cpumask_t accessibility;
> + atomic_t online_refs;
>
> /*
> * probe_lock is only taken during discovery. After discovery these
> @@ -223,6 +225,7 @@ struct mpam_msc_ris {
> u8 ris_idx;
> u64 idr;
> struct mpam_props props;
> + bool in_reset_state;
>
> cpumask_t affinity;
>
> @@ -242,6 +245,11 @@ struct mpam_msc_ris {
> extern struct srcu_struct mpam_srcu;
> extern struct list_head mpam_classes;
>
> +static inline void mpam_assert_srcu_read_lock_held(void)
> +{
> + WARN_ON_ONCE(!srcu_read_lock_held((&mpam_srcu)));
> +}
> +
> /* System wide partid/pmg values */
> extern u16 mpam_partid_max;
> extern u8 mpam_pmg_max;
Thanks,

Ben