linux-kernel - Re: [PATCH 02/15] x86/resctrl: Remove hard-coded memory bandwidth event configuration

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Date:   Wed, 6 Dec 2023 13:17:35 -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 02/15] x86/resctrl: Remove hard-coded memory bandwidth
 event configuration

Hi Reinette,

On 12/6/23 12:32, Reinette Chatre wrote:
> Hi Babu,
> 
> On 12/6/2023 9:17 AM, Moger, Babu wrote:
>> On 12/5/23 17:21, Reinette Chatre wrote:
>>> On 11/30/2023 4:57 PM, Babu Moger wrote:
> 
> ...
> 
>>> Comparing with supported bits would be an additional check, but what does
>>> that imply? Would it be possible for hardware to have a bit set that is
>>> not supported? Would that mean it is actually supported or a hardware bug?
>>
>> No. Hardware supports all the bits reported here. Like i said before
>> wanted to remove the hard-coded value.
> 
> The size of the field in the register is different information from what
> the value of that field may be.

Yes. it could be.

> 
> 
>>
>>>
>>>>  }
>>>>  
>>>>  static void mondata_config_read(struct rdt_domain *d, struct mon_config_info *mon_info)
>>>> @@ -1621,7 +1621,7 @@ static int mbm_config_write_domain(struct rdt_resource *r,
>>>>  	int ret = 0;
>>>>  
>>>>  	/* mon_config cannot be more than the supported set of events */
>>>> -	if (val > MAX_EVT_CONFIG_BITS) {
>>>> +	if (val > resctrl_max_evt_bitmask) {
>>>>  		rdt_last_cmd_puts("Invalid event configuration\n");
>>>>  		return -EINVAL;
>>>>  	}
>>>
>>> This does not look right. resctrl_max_evt_bitmask contains the supported
>>> types. A user may set a value that is less than resctrl_max_evt_bitmask but
>>> yet have an unsupported bit set, no?
>>
>> I think I have to make this clear in the patch. There is no difference in
>> the definition. Hardware supports all the events reported by the cpuid.
> 
> I'll try to elaborate using an example. Let's say AMD decides to make
> hardware with hypothetical support mask of:
> 	resctrl_max_evt_bitmask = 0x4F (no support for Slow Mem).
> 
> What if user attempts to set config that enables monitoring of Slow Mem:
> 	val = 0x30
> 
> In the above example, val is not larger than resctrl_max_evt_bitmask 
> but it is an invalid config, no?

Yes. It is invalid config in this case.

How about changing the check to something like this?

if ((val & resctrl_max_evt_bitmask) != val) {
                rdt_last_cmd_puts("Invalid event configuration\n");
  		return -EINVAL;
   }
-- 
Thanks
Babu Moger