From ca539a37d59a65d05538b24406a9ac49869fd86c Mon Sep 17 00:00:00 2001 From: Andre Przywara Date: Thu, 1 Oct 2026 17:34:35 +0200 Subject: [PATCH] arm_mpam: prepare mon_sel locking for MPAM-Fb The MSC MON_SEL register needs to be accessed from hardirq for the overflow interrupt, and when taking an IPI to access these registers on platforms where MSCs are not accesible from every CPU. This makes an irqsave spinlock the obvious lock to protect these registers. On systems with MPAM-Fb mailbox MSC access it must be able to sleep, meaning a mutex must be used. So MPAM-Fb platforms cannot support an overflow interrupt easily. Clearly these two methods can't exist for one MSC at the same time. Change the mon_sel locking wrapper function to only use a spinlock when the MSC is accessed directly via MMIO. In case of MPAM-Fb, we use a mutex, but only if we are in a sleepable context. If that's not the case, we return an error. This should not happen, as MPAM-Fb by design does not require an MSC access to happen from a specific CPU, so there is no need for any IPIs or preemption disabling to satisfy CPU constraints. And since overflow interrupts are not supported at the moment anyway, we also wouldn't meet the other case. Bailing out early is already happening in rare occasions today. Signed-off-by: Andre Przywara Reviewed-by: Jonathan Cameron Reviewed-by: Ben Horgan Reviewed-by: Gavin Shan Tested-by: Gavin Shan # on NVIDIA Grace Hopper Signed-off-by: James Morse --- drivers/resctrl/mpam_devices.c | 6 ++++- drivers/resctrl/mpam_internal.h | 43 +++++++++++++++++++++++++++------ 2 files changed, 41 insertions(+), 8 deletions(-) diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c index 31f4826ab399..2a0ac23cc042 100644 --- a/drivers/resctrl/mpam_devices.c +++ b/drivers/resctrl/mpam_devices.c @@ -2349,7 +2349,6 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev) if (err) return ERR_PTR(err); - mpam_mon_sel_lock_init(msc); msc->id = pdev->id; msc->pdev = pdev; INIT_LIST_HEAD_RCU(&msc->all_msc_list); @@ -2370,6 +2369,11 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev) else msc->iface = MPAM_IFACE_PCC; + /* Lock type depends on MSC interface used */ + err = mpam_mon_sel_lock_init(dev, msc); + if (err) + return ERR_PTR(err); + if (msc->iface == MPAM_IFACE_MMIO) { void __iomem *io; diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h index 09f8168fb4d2..58f7523bc087 100644 --- a/drivers/resctrl/mpam_internal.h +++ b/drivers/resctrl/mpam_internal.h @@ -126,6 +126,12 @@ struct mpam_msc { */ raw_spinlock_t _mon_sel_lock; unsigned long _mon_sel_flags; + /* + * mon_sel_mutex is the mutex version of the _mon_sel_lock above. + * Always use the mpam_mon_sel_lock() helpers when taking the lock, + * as this will select the correct lock type automatically. + */ + struct mutex mon_sel_mutex; void __iomem *mapped_hwpage; size_t mapped_hwpage_sz; @@ -139,27 +145,50 @@ struct mpam_msc { /* Returning false here means accesses to mon_sel must fail and report an error. */ static inline bool __must_check mpam_mon_sel_lock(struct mpam_msc *msc) { - /* Locking will require updating to support a firmware backed interface */ - if (WARN_ON_ONCE(msc->iface != MPAM_IFACE_MMIO)) + if (msc->iface == MPAM_IFACE_MMIO) { + raw_spin_lock_irqsave(&msc->_mon_sel_lock, msc->_mon_sel_flags); + + return true; + } + + if (!preemptible()) return false; - raw_spin_lock_irqsave(&msc->_mon_sel_lock, msc->_mon_sel_flags); + mutex_lock(&msc->mon_sel_mutex); + return true; } static inline void mpam_mon_sel_unlock(struct mpam_msc *msc) { - raw_spin_unlock_irqrestore(&msc->_mon_sel_lock, msc->_mon_sel_flags); + if (msc->iface == MPAM_IFACE_MMIO) { + raw_spin_unlock_irqrestore(&msc->_mon_sel_lock, + msc->_mon_sel_flags); + + return; + } + + mutex_unlock(&msc->mon_sel_mutex); } static inline void mpam_mon_sel_lock_held(struct mpam_msc *msc) { - lockdep_assert_held_once(&msc->_mon_sel_lock); + if (msc->iface == MPAM_IFACE_MMIO) + lockdep_assert_held_once(&msc->_mon_sel_lock); + else + lockdep_assert_held_once(&msc->mon_sel_mutex); } -static inline void mpam_mon_sel_lock_init(struct mpam_msc *msc) +static inline int mpam_mon_sel_lock_init(struct device *dev, + struct mpam_msc *msc) { - raw_spin_lock_init(&msc->_mon_sel_lock); + if (msc->iface == MPAM_IFACE_MMIO) { + raw_spin_lock_init(&msc->_mon_sel_lock); + + return 0; + } + + return devm_mutex_init(dev, &msc->mon_sel_mutex); } DEFINE_GUARD(mon_sel, struct mpam_msc *,