linux-kernel - Re: [PATCH v2 2/2] x86/resctrl: Remove hard-coded memory bandwidth event configuration

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Date: Wed, 3 Jan 2024 13:40:38 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <corbet@....net>, <fenghua.yu@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>
CC: <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
	<seanjc@...gle.com>, <kim.phillips@....com>, <jmattson@...gle.com>,
	<ilpo.jarvinen@...ux.intel.com>, <jithu.joseph@...el.com>,
	<kan.liang@...ux.intel.com>, <nikunj@....com>,
	<daniel.sneddon@...ux.intel.com>, <pbonzini@...hat.com>,
	<rick.p.edgecombe@...el.com>, <rppt@...nel.org>,
	<maciej.wieczor-retman@...el.com>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <eranian@...gle.com>,
	<peternewman@...gle.com>, <dhagiani@....com>
Subject: Re: [PATCH v2 2/2] x86/resctrl: Remove hard-coded memory bandwidth
 event configuration

Hi Babu,

On 1/3/2024 1:03 PM, Moger, Babu wrote:
> On 1/3/24 12:38, Reinette Chatre wrote:
>> On 1/2/2024 12:00 PM, Moger, Babu wrote:
>>> On 12/14/23 19:24, Reinette Chatre wrote:
>>>> On 12/12/2023 10:02 AM, Babu Moger wrote:
>>
>>>>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
>>>>> index f136ac046851..30bf919edfda 100644
>>>>> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
>>>>> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
>>>>> @@ -813,6 +813,12 @@ int __init rdt_get_mon_l3_config(struct rdt_resource *r)
>>>>>  		return ret;
>>>>>  
>>>>>  	if (rdt_cpu_has(X86_FEATURE_BMEC)) {
>>>>> +		u32 eax, ebx, ecx, edx;
>>>>> +
>>>>> +		/* Detect list of bandwidth sources that can be tracked */
>>>>> +		cpuid_count(0x80000020, 3, &eax, &ebx, &ecx, &edx);
>>>>> +		hw_res->event_mask = ecx;
>>>>> +
>>>>
>>>> This has the same issue as I mentioned in V1. Note that this treats
>>>> reserved bits as valid values. I think this is a risky thing to do. For example
>>>> when this code is run on future hardware the currently reserved bits may have
>>>> values with different meaning than what this code uses it for.
>>>
>>> Sure. Will use the mask MAX_EVT_CONFIG_BITS.
>>>               hw_res->mbm_cfg_mask = ecx &  MAX_EVT_CONFIG_BITS;
>>>
>>>>
>>>>>  		if (rdt_cpu_has(X86_FEATURE_CQM_MBM_TOTAL)) {
>>>>>  			mbm_total_event.configurable = true;
>>>>>  			mbm_config_rftype_init("mbm_total_bytes_config");
>>>>> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>>> index 69a1de92384a..8a1e9fdab974 100644
>>>>> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>>> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
>>>>> @@ -1537,17 +1537,14 @@ static void mon_event_config_read(void *info)
>>>>>  {
>>>>>  	struct mon_config_info *mon_info = info;
>>>>>  	unsigned int index;
>>>>> -	u64 msrval;
>>>>> +	u32 h;
>>>>>  
>>>>>  	index = mon_event_config_index_get(mon_info->evtid);
>>>>>  	if (index == INVALID_CONFIG_INDEX) {
>>>>>  		pr_warn_once("Invalid event id %d\n", mon_info->evtid);
>>>>>  		return;
>>>>>  	}
>>>>> -	rdmsrl(MSR_IA32_EVT_CFG_BASE + index, msrval);
>>>>> -
>>>>> -	/* Report only the valid event configuration bits */
>>>>> -	mon_info->mon_config = msrval & MAX_EVT_CONFIG_BITS;
>>>>> +	rdmsr(MSR_IA32_EVT_CFG_BASE + index, mon_info->mon_config, h);
>>>>
>>>> I do not think this code needed to be changed. We do not want to treat
>>>> reserved bits as valid values. 
>>>
>>> The logic is still the same. We don't have access to rdt_hw_resource in
>>> this function. So, I just moved the masking to mbm_config_show while printing.
>>
>> Why do you need access to rdt_hw_resource? This comment is not about the bandwidth
>> events supported by the device but instead the bits used to represent these events.
>> This is the same issue as in rdt_get_mon_l3_config. The above change returns
>> reserved bits as valid while the original code ensured that only bits used for
>> field are returned (through the usage of MAX_EVT_CONFIG_BITS).
> 
> We are already saving the valid bits in hw_res->mbm_cfg_mask during the init.
> 
> hw_res->mbm_cfg_mask = ecx &  MAX_EVT_CONFIG_BITS;
> 
> I thought we can use it here directly to mask any unsupported bits. So, I
> re-arranged the code here.

I am not sure where you mean when you say "use it here" since mbm_cfg_mask is not
used in mon_event_config_read(). My comment is related to mon_event_config_read()
that can reasonably be expected to, and thus should, return the current "mon event
config" value and nothing more. 

Reinette