linux-kernel - Re: [RFC PATCH 10/10] arch/x86: Introduce QOS feature for AMD

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Date:   Fri, 5 Oct 2018 17:18:35 +0000
From:   "Moger, Babu" <Babu.Moger@....com>
To:     James Morse <james.morse@....com>
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Subject: Re: [RFC PATCH 10/10] arch/x86: Introduce QOS feature for AMD

Hi James,

On 10/05/2018 11:20 AM, James Morse wrote:
> Hi Babu,
> 
> On 24/09/18 20:19, Moger, Babu wrote:
>> Enables QOS feature on AMD.
>> Following QoS sub-features are supported in AMD if the underlying
>> hardware supports it.
>>  - L3 Cache allocation enforcement
>>  - L3 Cache occupancy monitoring
>>  - L3 Code-Data Prioritization support
>>  - Memory Bandwidth Enforcement(Allocation)
>>
>> There are differences in the way some of the features are implemented.
>> Separate those functions and add those as vendor specific functions.
>> The major difference is in MBA feature.
>>  - AMD uses CPUID leaf 0x80000020 to initialize the MBA features.
>>  - AMD uses direct bandwidth value instead of delay based on bandwidth
>>    values.
>>  - MSR register base addresses are different for MBA.
> 
>>  - Also AMD allows non-contiguous L3 cache bit masks.
> 
> Nice!
> 
> This is visible to user-space, the 'Cache Bit Masks (CBM)' section of
> Documentation/x86/intel_rdt_ui.txt currently says 'X86 hardware requires ... a
> contiguous block'.
> 
> Does user-space need to know it can do this in advance, or is it a try-it-and-see?

It is try-it-and-see.
> 
> Arm's MPAM stuff can do this too, but I'm against having the ABI vary between
> architectures. If this is going to be discoverable, I'd like it to work on Arm too.

It not discoverable at this point. Mostly predefined. Yes. It will be bit
of a challenge handle these differences. We may have to come up with some
kind of a flag(or something) to make it look similar on the ABI side.

> 
> 
> Thanks,
> 
> James
> 
>> Adds following functions to take care of the differences.
>> rdt_get_mem_config_amd : MBA initialization function
>> parse_bw_amd : Bandwidth parsing
>> mba_wrmsr_amd: Writes bandwidth value
>> cbm_validate_amd : L3 cache bitmask validation
> 
>> diff --git a/arch/x86/kernel/cpu/rdt_ctrlmondata.c b/arch/x86/kernel/cpu/rdt_ctrlmondata.c
>> index 5a282b6c4bd7..1e4631f88696 100644
>> --- a/arch/x86/kernel/cpu/rdt_ctrlmondata.c
>> +++ b/arch/x86/kernel/cpu/rdt_ctrlmondata.c
>> @@ -123,6 +169,41 @@ bool cbm_validate(char *buf, u32 *data, struct rdt_resource *r)
>>  	return true;
>>  }
>>  
>> +/*
>> + * Check whether a cache bit mask is valid. AMD allows
>> + * non-contiguous masks.
>> + */
>> +bool cbm_validate_amd(char *buf, u32 *data, struct rdt_resource *r)
>> +{
>> +	unsigned long first_bit, zero_bit, val;
>> +	unsigned int cbm_len = r->cache.cbm_len;
>> +	int ret;
>> +
>> +	ret = kstrtoul(buf, 16, &val);
>> +	if (ret) {
>> +		rdt_last_cmd_printf("non-hex character in mask %s\n", buf);
>> +		return false;
>> +	}
>> +
>> +	if (val == 0 || val > r->default_ctrl) {
>> +		rdt_last_cmd_puts("mask out of range\n");
>> +		return false;
>> +	}
>> +
>> +	first_bit = find_first_bit(&val, cbm_len);
>> +	zero_bit = find_next_zero_bit(&val, cbm_len, first_bit);
>> +
>> +
>> +	if ((zero_bit - first_bit) < r->cache.min_cbm_bits) {
>> +		rdt_last_cmd_printf("Need at least %d bits in mask\n",
>> +				    r->cache.min_cbm_bits);
>> +		return false;
>> +	}
>> +
>> +	*data = val;
>> +	return true;
>> +}
>> +
>>  struct rdt_cbm_parse_data {
>>  	struct rdtgroup		*rdtgrp;
>>  	char			*buf;
>>
>