Re: [RFC PATCH 1/3] arm64: add CPU prefetch and cache modulation sysfs interface

[Rob Herring <robh@kernel.org>]

On Mon, Aug 17, 2026 at 10:23:33AM +0800, KobaK wrote:
> From: Koba Ko <kobak@nvidia.com>
> 
> Performance characterization on arm64 systems sometimes needs a way to
> inspect and bracket selected implementation-defined prefetch and cache
> controls without exposing raw register contents. Add an RFC-only arm64
> cpumod interface that presents a small set of named, range-checked
> per-CPU sysfs attributes under each CPU device.
> 
> Establish the Kconfig and Makefile plumbing, CPU profile detection for
> Grace and Vera from the target CPU's MIDR, register-field descriptors,
> per-CPU kobject and attribute helpers, locking, and target-CPU read/write
> callbacks. Common attributes are exposed for all supported profiles,
> while Grace- and Vera-specific fields are exposed only for the matching
> profile. Place the option at the end of the top-level Kernel Features
> menu because these implementation controls are not tied to an Arm
> architecture revision.
> 
> Per-CPU cpumod sysfs state follows the CPU lifecycle. Module
> initialization populates subtrees for every online CPU while holding
> cpus_read_lock(), so initial enumeration and CPU hotplug state
> registration are serialized against concurrent topology changes. Offline
> CPUs are intentionally skipped during initial setup because target-CPU
> MIDR detection requires an IPI-capable online CPU; a later online callback
> creates their subtree.
> 
> Profile detection reads MIDR through a synchronous callback on the CPU
> owning the sysfs subtree and returns nonzero only when that dispatch
> fails, so no IPI error can be hidden. After a successful read, retain
> CPUMOD_PROFILE_UNKNOWN as the unsupported-profile sentinel. Subtree
> creation skips that profile during both initial online enumeration and
> later CPUHP online callbacks without blocking module load or CPU hotplug.
> Object and sysfs setup failures remain fatal and are propagated.
> 
> Keep the dynamically allocated kobject state in a private per-CPU pointer
> instead of the CPU device's generic driver-data slot. The online callback
> is idempotent when that pointer already records a subtree. The offline
> callback clears the private pointer before dropping the kobject reference,
> allowing the release callback to retain ownership of the final free.
> 
> Register the dynamic CPUHP state with
> cpuhp_setup_state_nocalls_cpuslocked(). The nocalls form avoids replaying
> startup callbacks for CPUs already initialized by the explicit online
> enumeration. On init failure, release the CPU read lock, destroy all
> subtrees created so far, and return the error. Module exit removes the
> CPUHP state with the matching nocalls helper and tears down all remaining
> per-CPU state across possible CPUs.
> 
> Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>
> Reviewed-by: Tushar Dave <tdave@nvidia.com>
> Reviewed-by: Kai-Heng Feng <kaihengf@nvidia.com>
> Signed-off-by: Koba Ko <kobak@nvidia.com>
> ---
>  arch/arm64/Kconfig                      |  17 +
>  arch/arm64/kernel/Makefile              |   1 +
>  arch/arm64/kernel/arm_cpumod.c          | 471 ++++++++++++++++++++++++
>  arch/arm64/kernel/arm_cpumod_internal.h | 113 ++++++
>  4 files changed, 602 insertions(+)
>  create mode 100644 arch/arm64/kernel/arm_cpumod.c
>  create mode 100644 arch/arm64/kernel/arm_cpumod_internal.h
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b3afe0688919b..5e4ada4884b89 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2417,6 +2417,23 @@ config ARM64_CONTPTE
>  	  bit, for any mappings that meet the size and alignment requirements.
>  	  This reduces TLB pressure and improves performance.
>  
> +config ARM64_CPUMOD
> +	tristate "Arm CPU prefetch modulation sysfs controls"
> +	help
> +	  Expose selected CPU implementation control register fields through
> +	  per-CPU sysfs attributes for performance characterization.
> +
> +	  The exposed controls allow hardware prefetch and cache-management
> +	  policy fields to be inspected and adjusted on CPUs where firmware
> +	  permits EL1 access to the relevant implementation control registers.
> +	  Systems where firmware traps or blocks these writes cannot use this
> +	  interface.
> +
> +	  This interface is intended for controlled performance evaluation and
> +	  should not be enabled by default on production systems.
> +
> +	  If unsure, say N.
> +
>  endmenu # "Kernel Features"
>  
>  menu "Boot options"
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index d2690c3ec5288..f180a52284e7d 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -69,6 +69,7 @@ obj-$(CONFIG_VMCORE_INFO)		+= vmcore_info.o
>  obj-$(CONFIG_ARM_SDE_INTERFACE)		+= sdei.o
>  obj-$(CONFIG_ARM64_PTR_AUTH)		+= pointer_auth.o
>  obj-$(CONFIG_ARM64_MPAM)		+= mpam.o
> +obj-$(CONFIG_ARM64_CPUMOD)		+= arm_cpumod.o
>  obj-$(CONFIG_ARM64_MTE)			+= mte.o
>  obj-y					+= vdso-wrap.o
>  obj-$(CONFIG_COMPAT_VDSO)		+= vdso32-wrap.o
> diff --git a/arch/arm64/kernel/arm_cpumod.c b/arch/arm64/kernel/arm_cpumod.c
> new file mode 100644
> index 0000000000000..9e64f2c8e7810
> --- /dev/null
> +++ b/arch/arm64/kernel/arm_cpumod.c
> @@ -0,0 +1,471 @@
> +// SPDX-License-Identifier: GPL-2.0-only

Since the introduction of this...

> +/*
> + * Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES.  All rights reserved.
> + *
> + * This program is free software; you can redistribute it and/or modify it
> + * under the terms and conditions of the GNU General Public License,
> + * version 2, as published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> + * more details.

We stopped putting license text in.

Before you go work on my comments, I'd wait for arm64 maintainers 
comments. I doubt this whole thing will be accepted. In general, we 
don't put in impdef interfaces to userspace.

Rob