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

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Message-ID: <7690b121-dd54-ea58-ec77-2dbb88c25b16@intel.com>
Date:   Tue, 2 Oct 2018 15:13:02 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     "Moger, Babu" <Babu.Moger@....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>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "kstewart@...uxfoundation.org" <kstewart@...uxfoundation.org>,
        "bp@...e.de" <bp@...e.de>,
        "rafael.j.wysocki@...el.com" <rafael.j.wysocki@...el.com>,
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        "xiaochen.shen@...el.com" <xiaochen.shen@...el.com>,
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        "Hurwitz, Sherry" <sherry.hurwitz@....com>,
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        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 10/10] arch/x86: Introduce QOS feature for AMD

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.

> +
> +	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?

Reinette