Re: [PATCH v12 03/26] x86/cpufeatures: Add support for Assignable Bandwidth Monitoring Counters (ABMC)

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Babu,

On 4/15/25 12:43 PM, Moger, Babu wrote:
> Hi Reinette,
> 
> On 4/15/25 11:09, Reinette Chatre wrote:
>> Hi Babu,
>>
>> On 4/14/25 10:48 AM, Moger, Babu wrote:
>>
>>> Here is my proposal to handle this case. This can be separate patch.
>>>
>>>
>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> index d10cf1e5b914..772f2f77faee 100644
>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>> @@ -1370,7 +1370,7 @@ static int rdt_mon_features_show(struct
>>> kernfs_open_file *of,
>>>
>>>         list_for_each_entry(mevt, &r->mon.evt_list, list) {
>>>                 seq_printf(seq, "%s\n", mevt->name);
>>> -               if (mevt->configurable)
>>> +               if (mevt->configurable &&
>>> !resctrl_arch_mbm_cntr_assign_enabled(r))
>>>                         seq_printf(seq, "%s_config\n", mevt->name);
>>>         }
>>>
>>> @@ -1846,6 +1846,11 @@ static int mbm_config_show(struct seq_file *s,
>>> struct rdt_resource *r, u32 evtid
>>>         cpus_read_lock();
>>>         mutex_lock(&rdtgroup_mutex);
>>>
>>> +       if (resctrl_arch_mbm_cntr_assign_enabled(r)) {
>>> +               rdt_last_cmd_puts("Event configuration(BMEC) not supported
>>> with mbm_cntr_assign mode\n");
>>> +               return -EINVAL;
>>> +       }
>>> +
>>>         list_for_each_entry(dom, &r->mon_domains, hdr.list) {
>>>                 if (sep)
>>>                         seq_puts(s, ";");
>>> @@ -1865,21 +1870,24 @@ static int mbm_config_show(struct seq_file *s,
>>> struct rdt_resource *r, u32 evtid
>>>  static int mbm_total_bytes_config_show(struct kernfs_open_file *of,
>>>                                        struct seq_file *seq, void *v)
>>>  {
>>> +       int ret;
>>>         struct rdt_resource *r = of->kn->parent->priv;
>>>
>>> -       mbm_config_show(seq, r, QOS_L3_MBM_TOTAL_EVENT_ID);
>>> +       ret = mbm_config_show(seq, r, QOS_L3_MBM_TOTAL_EVENT_ID);
>>>
>>> -       return 0;
>>> +       return ret;
>>>  }
>>>
>>>  static int mbm_local_bytes_config_show(struct kernfs_open_file *of,
>>>                                        struct seq_file *seq, void *v)
>>>  {
>>> +       int ret;
>>> +
>>>         struct rdt_resource *r = of->kn->parent->priv;
>>>
>>> -       mbm_config_show(seq, r, QOS_L3_MBM_LOCAL_EVENT_ID);
>>> +       ret = mbm_config_show(seq, r, QOS_L3_MBM_LOCAL_EVENT_ID);
>>>
>>> -       return 0;
>>> +       return ret;
>>>  }
>>>
>>>  static void mbm_config_write_domain(struct rdt_resource *r,
>>> @@ -1932,6 +1940,11 @@ static int mon_config_write(struct rdt_resource *r,
>>> char *tok, u32 evtid)
>>>         /* Walking r->domains, ensure it can't race with cpuhp */
>>>         lockdep_assert_cpus_held();
>>>
>>> +       if (resctrl_arch_mbm_cntr_assign_enabled(r)) {
>>> +               rdt_last_cmd_puts("Event configuration(BMEC) not supported
>>> with mbm_cntr_assign mode\n");
>>> +               return -EINVAL;
>>> +       }
>>> +
>>>  next:
>>>         if (!tok || tok[0] == '\0')
>>>                 return 0;
>>>
>>
>> Instead of chasing every call that may involve BMEC I think it will be simpler to
>> disable BMEC support during initialization when ABMC is detected. Specifically,
>> on systems that support both BMEC and ABMC rdt_cpu_has(X86_FEATURE_BMEC) returns
>> false. 
> 
> There is one problem with this approach. Users have the option to switch
> between the assignment modes. System will boot with ABMC by default if
> supported. But, users can switch to 'default' mode after the boot. By
> disabling the BMEC completely, it will not be possible to do that.

Good point. Thank you. Another option is to hide (see kernfs_show()) mbm_total_bytes_config
and mbm_local_bytes_config when ABMC is enabled. To me this seems like a clear
interface to user space, when user interface changes the mode the interface changes
to reflect new mode.

> 
>>
>> I would also like to consider enhancing mevt->configurable to handle all different
>> ways in which events can be configured. For example, making mevt->configurable an
>> enum that captures how event can be configured instead of keeping mevt->configurable
>> a boolean for BMEC support and handling ABMC completely separately. I hope this
>> may become clearer when using struct mon_evt for ABMC also.
> 
> Sure. I can try that.

Thank you.

Reinette