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Date: Thu, 4 Jan 2024 07:48:53 -0600
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Reply-To: babu.moger@amd.com
Subject: Re: [PATCH v2 2/2] x86/resctrl: Remove hard-coded memory bandwidth
 event configuration
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To: Reinette Chatre <reinette.chatre@intel.com>, corbet@lwn.net,
 fenghua.yu@intel.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
 dave.hansen@linux.intel.com
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 maciej.wieczor-retman@intel.com, linux-doc@vger.kernel.org,
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From: "Moger, Babu" <babu.moger@amd.com>
In-Reply-To: <54c18d29-3925-4bb6-bf39-ec5cb6dfeedf@intel.com>
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Hi Reinette,

On 1/3/24 15:40, Reinette Chatre wrote:
> 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. 
> 

Ok. Sure. Lets keep the same code as before.

-- 
Thanks
Babu Moger