Date: Fri, 10 Mar 2017 13:51:46 -0800 (PST)
From: Shivappa Vikas <vikas.shivappa@linux.intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
cc: Vikas Shivappa <vikas.shivappa@linux.intel.com>,
        vikas.shivappa@intel.com, linux-kernel@vger.kernel.org, x86@kernel.org,
        hpa@zytor.com, mingo@kernel.org, peterz@infradead.org,
        ravi.v.shankar@intel.com, tony.luck@intel.com, fenghua.yu@intel.com,
        andi.kleen@intel.com
Subject: Re: [PATCH 4/8] x86/intel_rct/mba: Add MBA structures and initialize
 MBA
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References: <1487361535-9727-1-git-send-email-vikas.shivappa@linux.intel.com> <1487361535-9727-5-git-send-email-vikas.shivappa@linux.intel.com> <alpine.DEB.2.20.1703011541070.4005@nanos>
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On Wed, 1 Mar 2017, Thomas Gleixner wrote:

> On Fri, 17 Feb 2017, Vikas Shivappa wrote:
>> --- a/arch/x86/include/asm/intel_rdt.h
>> +++ b/arch/x86/include/asm/intel_rdt.h
>> @@ -11,6 +11,9 @@
>>  #define IA32_L3_QOS_CFG		0xc81
>>  #define IA32_L3_CBM_BASE	0xc90
>>  #define IA32_L2_CBM_BASE	0xd10
>> +#define IA32_MBA_THRTL_BASE	0xd50
>> +#define MAX_MBA_THRTL		100u
>> +#define MBA_IS_LINEAR		0x4
>
> I have a hard time to figure out how the latter two constants are related
> to this list of registers. MBA_IS_LINEAR is used to check the CPUID bit and
> MAX_MBA_THRTL is obviously a pure software constant because with a
> non-linear scale the maximum value is not 100.
>
> Just slapping defines to random places is equally bad as using hard coded
> constants.
>
>> +/*
>> + * rdt_get_mb_table() - get a mapping of b/w percentage values
>> + * exposed to user interface and the h/w understandable delay values.
>> + *
>> + * The non-linear delay values have the granularity of power of two
>> + * and also the h/w does not guarantee a curve for configured delay
>> + * values vs. actual b/w throttled.
>> + * Hence we need a mapping that is pre caliberated for user to express
>> + * the b/w in terms of any sensible number.
>
> ... calibrated so the user can express the bandwidth as a percentage value.
>
>> +static inline int rdt_get_mb_table(struct rdt_resource *r)
>> +{
>> +	/*
>> +	 * There are no Intel SKUs as of now to support non-linear delay.
>> +	 */
>> +	r->mb_map = NULL;
>
> What's the point of setting this to NULL?
>
> Also it would be helpful to emit log info here so people don't have to
> start digging around.
>
> 	pr_info("Bandwidth map not implemented for ....", ... model);
>
>> +
>> +	return -ENODEV;
>
> Returning -ENODEV to a function which just returns a boolean value is
> pointless.
>
>>  static void rdt_get_cache_config(int idx, struct rdt_resource *r)
>>  {
>>  	union cpuid_0x10_1_eax eax;
>> @@ -184,9 +237,8 @@ static inline bool get_rdt_resources(void)
>>  		ret = true;
>>  	}
>>
>> -	if (boot_cpu_has(X86_FEATURE_MBA)) {
>> -		ret = true;
>> -	}
>> +	if (boot_cpu_has(X86_FEATURE_MBA))
>> +		ret = rdt_get_mem_config(&rdt_resources_all[RDT_RESOURCE_MBA]);
>
> Groan. When rdt_get_mem_config() returns false (because the map is not
> implemented), then the whole function returns false and CAT is disabled.
>
>> +static int domain_setup_ctrlval(struct rdt_resource *r, struct rdt_domain *d)
>> +{
>> +	int i;
>> +
>> +	d->ctrl_val = kmalloc_array(r->num_closid,
>> +				     sizeof(*d->ctrl_val), GFP_KERNEL);
>> +	if (!d->ctrl_val)
>> +		return -ENOMEM;
>> +
>> +	/*
>> +	 * Initialize the Control MSRs to having no control.
>> +	 * For Cache Allocation: Set all bits in cbm
>> +	 * For Memory Allocation: Set b/w requested to 100
>> +	 */
>> +	for (i = 0; i < r->num_closid; i++) {
>> +		int idx = cbm_idx(r, i);
>> +
>> +		d->ctrl_val[i] = r->default_ctrl;
>> +		wrmsrl(r->msr_base + idx, d->ctrl_val[i]);
>> +	}
>
> So if you use a local pointer for that, this whole mess becomes readable.
>
> static int domain_setup_ctrlval(struct rdt_resource *r, struct rdt_domain *d)
> {
> 	u32 *p;
> 	int i;
>
> 	p = kmalloc_array(r->num_closid, sizeof(*d->ctrl_val), GFP_KERNEL);
> 	if (!p)
> 		return -ENOMEM;
>
> 	d->ctrl_val = p;
>
> 	/* Initialize the Control MSRs to the default value */
> 	for (i = 0; i < r->num_closid; i++, p++) {
> 		int idx = cbm_idx(r, i);
>
> 		*p = r->default_ctrl;
> 		wrmsrl(r->msr_base + idx, *p);
> 	}
>> +
>> +	return 0;
>> +}
>
>>  static void domain_add_cpu(int cpu, struct rdt_resource *r)
>>  {
>> -	int i, id = get_cache_id(cpu, r->cache_level);
>> +	int id = get_cache_id(cpu, r->cache_level), ret;
>
> Bah. If you have the same type in one line, then please move the
> uninitialized variables to the front.
>
> 	int ret, id = get_cache_id(cpu, r->cache_level);
>
> But a s/i/ret/ would have been to simple and kept the code readable.
>
>> @@ -298,19 +374,12 @@ static void domain_add_cpu(int cpu, struct rdt_resource *r)
>>
>>  	d->id = id;
>>
>> -	d->ctrl_val = kmalloc_array(r->num_closid, sizeof(*d->ctrl_val), GFP_KERNEL);
>> -	if (!d->ctrl_val) {
>> +	ret = domain_setup_ctrlval(r, d);
>> +	if (ret) {
>>  		kfree(d);
>>  		return;
>>  	}
>
> What's the point of this 'ret' variable if the function is void?
>
> 	if (domain_setup_ctrlval(r, d)) {
> 		kfree(d);
> 		return;
> 	}
>
> would have been to easy to read, right?

Will fix all the issues pointed. Thanks for pointing out.

>
> Thanks,
>
> 	tglx
>