Re: [PATCH RFC v5 03/18] riscv: add support for srmcfg CSR from Ssqosid extension

[Drew Fustini <fustini@kernel.org>]

On Mon, May 25, 2026 at 12:30:07AM +0000, sashiko-bot@kernel.org wrote:
> > +static int riscv_srmcfg_pm_notify(struct notifier_block *nb,
> > +				  unsigned long action, void *unused)
> > +{
> > +	switch (action) {
> > +	case CPU_PM_EXIT:
> > +	case CPU_PM_ENTER_FAILED:
> > +		__this_cpu_write(cpu_srmcfg, U32_MAX);
> > +		break;
> > +	}
> > +	return NOTIFY_OK;
> > +}
> 
> [Severity: High]
> Does this leave the currently executing task with an incorrect hardware CSR
> state?
> 
> While the software cache is invalidated by setting cpu_srmcfg to U32_MAX,
> the hardware CSR_SRMCFG register is not explicitly reprogrammed during
> CPU_PM_EXIT.
> 
> Since the hardware state may be lost or reset to an implementation-defined
> value upon resume, wouldn't the task running at resume time continue executing
> with the wrong QoS tag until it is eventually preempted?

This is not a problem. cache invalidation is sufficient. CSR write here
would be setting a value that __switch_to_srmcfg() will re-write moments
later.

> > +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_online, NULL);
> 
> [Severity: High]
> Should a teardown callback be provided to cpuhp_setup_state() to invalidate
> the cpu_srmcfg cache when a CPU goes offline?

The startup callback riscv_srmcfg_online() is the one that invalidates
the cache. A teardown callback that did the same thing on offline would be
redundant with the online seed. 

> Without a teardown callback, a CPU going offline retains its previous
> cpu_srmcfg value. When brought back online, its hardware CSR is reset.
> 
> Tasks scheduling before the CPU reaches the CPUHP_AP_ONLINE_DYN state might
> skip the CSR write if their thread srmcfg tag matches the stale cache. Could
> this allow those early context switches to bypass resource constraints by
> running with the hardware-reset CSR?

I will move the seed earlier to CPUHP_AP_STARTING so the cache is
invalidated before the CPU is scheduleable.

-Drew