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

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Message-ID: <d748a754-6c46-45b4-8e62-8a0775565c10@amd.com>
Date: Thu, 17 Apr 2025 09:27:41 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, tony.luck@...el.com,
 peternewman@...gle.com
Cc: corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 paulmck@...nel.org, akpm@...ux-foundation.org, thuth@...hat.com,
 rostedt@...dmis.org, ardb@...nel.org, gregkh@...uxfoundation.org,
 daniel.sneddon@...ux.intel.com, jpoimboe@...nel.org,
 alexandre.chartre@...cle.com, pawan.kumar.gupta@...ux.intel.com,
 thomas.lendacky@....com, perry.yuan@....com, seanjc@...gle.com,
 kai.huang@...el.com, xiaoyao.li@...el.com, kan.liang@...ux.intel.com,
 xin3.li@...el.com, ebiggers@...gle.com, xin@...or.com,
 sohil.mehta@...el.com, andrew.cooper3@...rix.com, mario.limonciello@....com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 maciej.wieczor-retman@...el.com, eranian@...gle.com
Subject: Re: [PATCH v12 03/26] x86/cpufeatures: Add support for Assignable
 Bandwidth Monitoring Counters (ABMC)

Hi Reinette,

On 4/16/25 11:08, Reinette Chatre wrote:
> 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.

Sure. Will try this. Thanks for the pointer.

-- 
Thanks
Babu Moger