Re: [PATCH v6 3/8] riscv: Add support for srmcfg CSR from Ssqosid extension

[Paul Walmsley <pjw@kernel.org>]

Hi,

On Wed, 29 Jul 2026, Drew Fustini wrote:

> Add support for the srmcfg CSR defined in the Ssqosid ISA extension.
> The CSR contains two fields:
> 
>   - Resource Control ID (RCID) for resource allocation
>   - Monitoring Counter ID (MCID) for tracking resource usage
> 
> Requests from a hart to shared resources are tagged with these IDs,
> allowing resource usage to be associated with the running task.
> 
> Add a srmcfg field to thread_struct with the same format as the CSR.
> The context-switch path writes the field to the CSR, and
> resctrl_arch_set_closid_rmid() updates it when a task is assigned to a
> resctrl control or monitoring group.
> 
> A per-cpu cpu_srmcfg_default holds the default srmcfg for each CPU, set
> by resctrl_arch_set_cpu_default_closid_rmid() on CPU group assignment.
> On context switch, RCID and MCID inherit from the CPU default
> independently: a task whose thread RCID field is zero takes the CPU
> default's RCID, and likewise for MCID.

Thanks, queued for v7.3, but with one change (below):

> diff --git a/arch/riscv/include/asm/qos.h b/arch/riscv/include/asm/qos.h
> new file mode 100644
> index 000000000000..cf19e8438bb9
> --- /dev/null
> +++ b/arch/riscv/include/asm/qos.h
> @@ -0,0 +1,74 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_RISCV_QOS_H
> +#define _ASM_RISCV_QOS_H
> +
> +#include <linux/percpu-defs.h>
> +
> +#ifdef CONFIG_RISCV_ISA_SSQOSID
> +
> +#include <linux/bitfield.h>
> +#include <linux/cpufeature.h>
> +#include <linux/sched.h>
> +
> +#include <asm/csr.h>
> +#include <asm/hwcap.h>
> +
> +/* cached value of srmcfg csr for each cpu */
> +DECLARE_PER_CPU(u32, cpu_srmcfg);
> +
> +/* default srmcfg value for each cpu, set via resctrl cpu assignment */
> +DECLARE_PER_CPU(u32, cpu_srmcfg_default);
> +
> +static inline void __switch_to_srmcfg(struct task_struct *next)
> +{
> +	u32 thread_srmcfg, default_srmcfg;
> +
> +	thread_srmcfg = READ_ONCE(next->thread.srmcfg);

What is the intent of the READ_ONCE() here?  

I've dropped it from what's been queued for now.  If it's really 
needed, please let us know why.


thanks,

- Paul