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

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Date:   Wed, 6 Dec 2023 10:32:03 -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 02/15] x86/resctrl: Remove hard-coded memory bandwidth
 event configuration

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.


> 
>>
>>>  }
>>>  
>>>  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?

Reinette