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

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Message-ID: <94e59101-8aa5-01f3-afaa-71701cc78cd5@amd.com>
Date:   Wed, 3 Oct 2018 17:21:18 +0000
From:   "Moger, Babu" <Babu.Moger@....com>
To:     Reinette Chatre <reinette.chatre@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "fenghua.yu@...el.com" <fenghua.yu@...el.com>,
        "vikas.shivappa@...ux.intel.com" <vikas.shivappa@...ux.intel.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "pombredanne@...b.com" <pombredanne@...b.com>,
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Subject: Re: [RFC PATCH 10/10] arch/x86: Introduce QOS feature for AMD

Hi Reinette,

On 10/02/2018 05:13 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 9/24/2018 12:19 PM, Moger, Babu wrote:
>> +/*
>> + * 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;
>> +	}
> 
> According to
> https://www.amd.com/system/files/TechDocs/56375_Quality_of_Service_Extensions.pdf
> "If an L3_MASK_n register is programmed with all 0’s, that COS will be
> prevented from allocating any lines in the L3 cache."
> 
> The "val == 0" test thus does not seem necessary.

Yes. Good point. We don't need this test.
> 
>> +
>> +	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;
>> +	}
> 
> If AMD platforms accept CBM of all zeroes then it seems that the
> platforms would not require a minimum number of set bits?

Yes. We don't need this check as well.  Tested and confirmed.
Thanks
> 
> Reinette
>