Re: [PATCH v13 04/25] x86/resctrl: Centralize monitoring feature enumeration

[Chen Yu]

Hi Tony,

On Mon, Sep 28, 2026 at 03:14:48PM -0700, Tony Luck wrote:
> The original implementation of Intel Cache QoS Monitoring (CQM) planned
> to integrate with the "perf" and "cgroup" subsystems. With that plan it
> made sense for parameters from CPUID to be stored in fields of the
> cpuinfo_x86 structure. But that plan was abandoned and the resctrl file
> system user interface replaced it.
> 
> Centralize all L3 monitoring enumeration within resctrl in preparation for
> removal of resctrl fields from struct cpuinfo_x86.
> 
> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>

[ ... ]

> @@ -132,11 +133,9 @@ static inline void __resctrl_sched_in(struct task_struct *tsk)
>  
>  static inline unsigned int resctrl_arch_round_mon_val(unsigned int val)
>  {
> -	unsigned int scale = boot_cpu_data.x86_cache_occ_scale;
> -
> -	/* h/w works in units of "boot_cpu_data.x86_cache_occ_scale" */
> -	val /= scale;
> -	return val * scale;
> +	/* Round down to nearest h/w monitoring unit */
> +	val /= rdt_l3_mon_scale;

I wonder if we should take this chance to check if rdt_l3_mon_scale is
0 on broken hardware?

> +	return val * rdt_l3_mon_scale;
>  }
>  
> + */
> +unsigned int __ro_after_init rdt_l3_mon_scale;
> +
>  #define CF(cf)	((unsigned long)(1048576 * (cf) + 0.5))
>  
>  static int snc_nodes_per_l3_cache = 1;
> @@ -418,16 +423,37 @@ static __init int snc_get_config(void)
>  
>  int __init rdt_get_l3_mon_config(struct rdt_resource *r)
>  {
> -	unsigned int mbm_offset = boot_cpu_data.x86_cache_mbm_width_offset;
>  	struct rdt_hw_resource *hw_res = resctrl_to_arch_res(r);
> +	unsigned int mbm_offset;
>  	unsigned int threshold;
>  	u32 eax, ebx, ecx, edx;
> +	u32 num_rmid;
> +
> +	/* Resource monitoring leaf is 0xf. L3 monitoring details in subleaf 1 */
> +	cpuid_count(0xf, 1, &eax, &ebx, &ecx, &edx);
> +	mbm_offset = eax & GENMASK(7, 0);
> +	rdt_l3_mon_scale = ebx;
>

Sashiko warns me that ebx might be 0, would 
rdt_l3_mon_scale = max(ebx, 1);
be doable?

thanks,
Chenyu