Re: [PATCH v2 3/8] riscv: Add support for srmcfg CSR from Ssqosid extension

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] CPU hotplug cache invalidation is registered at the wrong state (CPUHP_AP_ONLINE_DYN), allowing the srmcfg cache sentinel to be overwritten before the CPU goes offline.
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commit d51eb8c4a6c5a57f238cc53688229c08ea6418e1
Author: Drew Fustini <fustini@xxxxxxxxxx>

riscv: Add support for srmcfg CSR from Ssqosid extension

This commit adds support for the srmcfg CSR defined in the Ssqosid ISA
extension. It introduces a per-cpu cache to avoid redundant CSR writes
during context switches and uses CPU hotplug and PM notifiers to manage
cache invalidation.

Link: https://github.com/riscv/riscv-ssqosid/releases/tag/v1.0

> diff --git a/arch/riscv/kernel/qos.c b/arch/riscv/kernel/qos.c
> new file mode 100644
> index 0000000000000..ea33201a43f61
> --- /dev/null
> +++ b/arch/riscv/kernel/qos.c

[ ... ]

> +static int riscv_srmcfg_reset_cache(unsigned int cpu)
> +{
> +	per_cpu(cpu_srmcfg, cpu) = U32_MAX;
> +	return 0;
> +}

[ ... ]

> +static int __init riscv_srmcfg_init(void)
> +{
> +	int err;
> +
> +	if (!riscv_has_extension_unlikely(RISCV_ISA_EXT_SSQOSID))
> +		return 0;
> +
> +	/*
> +	 * cpuhp_setup_state() invokes the startup callback locally on every
> +	 * already-online CPU, so no separate seed loop is needed here.
> +	 */
> +	err = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "riscv/srmcfg:online",
> +				riscv_srmcfg_reset_cache, riscv_srmcfg_reset_cache);

[Severity: High]
Does registering the hotplug callbacks at CPUHP_AP_ONLINE_DYN run too early
during teardown and too late during startup?

Since CPUHP_AP_ONLINE_DYN teardown runs while the CPU is still schedulable,
could the U32_MAX sentinel set by riscv_srmcfg_reset_cache() be overwritten
by subsequent context switches (such as switching to the idle thread) before
the CPU actually goes offline?

If a CPU is offlined and then onlined, the stale cache would persist while
the hardware CSR has been reset. Because the CPU becomes schedulable before
the CPUHP_AP_ONLINE_DYN startup callback runs, a task matching the stale
cache might skip the required CSR write and run with incorrect hardware tags.

> +	if (err < 0) {
> +		pr_warn("srmcfg cpuhp registration failed (%d), cpus brought online after boot will not invalidate the CSR_SRMCFG cache\n",
> +			err);
> +		return err;
> +	}
> +
> +	cpu_pm_register_notifier(&riscv_srmcfg_pm_nb);
> +	return 0;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260624-dfustini-atl-sc-cbqri-dt-v2-0-2f8049fd902b@xxxxxxxxxx?part=3