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

Moger, Babu
2025-04-17T14:27:48Z

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