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 15:03:03 -0600
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.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 Reinette,

On 1/3/24 12:38, Reinette Chatre wrote:
> Hi Babu,
> 
> 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.
-- 
Thanks
Babu Moger