Subject: Re: [PATCH v6 05/12] ACPI/PPTT: Add Processor Properties Topology
 Table parsing
To: Sudeep Holla <sudeep.holla@arm.com>
Cc: linux-acpi@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        hanjun.guo@linaro.org, lorenzo.pieralisi@arm.com,
        rjw@rjwysocki.net, will.deacon@arm.com, catalin.marinas@arm.com,
        gregkh@linuxfoundation.org, viresh.kumar@linaro.org,
        mark.rutland@arm.com, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, jhugo@codeaurora.org,
        wangxiongfeng2@huawei.com, Jonathan.Zhang@cavium.com,
        ahs3@redhat.com, Jayachandran.Nair@cavium.com,
        austinwc@codeaurora.org, lenb@kernel.org, vkilari@codeaurora.org,
        morten.rasmussen@arm.com
References: <20180113005920.28658-1-jeremy.linton@arm.com>
 <20180113005920.28658-6-jeremy.linton@arm.com>
 <20180115145839.GA12531@e107155-lin>
From: Jeremy Linton <jeremy.linton@arm.com>
Message-ID: <65c42ed6-8ac7-3adb-43f2-de20080eaaa9@arm.com>
Date: Tue, 16 Jan 2018 14:55:56 -0600
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Hi,

On 01/15/2018 08:58 AM, Sudeep Holla wrote:
> On Fri, Jan 12, 2018 at 06:59:13PM -0600, Jeremy Linton wrote:
>> ACPI 6.2 adds a new table, which describes how processing units
>> are related to each other in tree like fashion. Caches are
>> also sprinkled throughout the tree and describe the properties
>> of the caches in relation to other caches and processing units.
>>
>> Add the code to parse the cache hierarchy and report the total
>> number of levels of cache for a given core using
>> acpi_find_last_cache_level() as well as fill out the individual
>> cores cache information with cache_setup_acpi() once the
>> cpu_cacheinfo structure has been populated by the arch specific
>> code.
>>
>> An additional patch later in the set adds the ability to report
>> peers in the topology using find_acpi_cpu_topology()
>> to report a unique ID for each processing unit at a given level
>> in the tree. These unique id's can then be used to match related
>> processing units which exist as threads, COD (clusters
>> on die), within a given package, etc.
>>
>> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
>> ---
>>   drivers/acpi/pptt.c | 476 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 476 insertions(+)
>>   create mode 100644 drivers/acpi/pptt.c
>>
>> diff --git a/drivers/acpi/pptt.c b/drivers/acpi/pptt.c
>> new file mode 100644
>> index 000000000000..2c4b3ed862a8
>> --- /dev/null
>> +++ b/drivers/acpi/pptt.c
>> @@ -0,0 +1,476 @@
>> +/*
>> + * Copyright (C) 2018, ARM
>> + *
>> + * This program is free software; you can redistribute it and/or modify it
>> + * under the terms and conditions of the GNU General Public License,
>> + * version 2, as published by the Free Software Foundation.
>> + *
>> + * This program is distributed in the hope it will be useful, but WITHOUT
>> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
>> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
>> + * more details.
>> + *
>> + * This file implements parsing of Processor Properties Topology Table (PPTT)
>> + * which is optionally used to describe the processor and cache topology.
>> + * Due to the relative pointers used throughout the table, this doesn't
>> + * leverage the existing subtable parsing in the kernel.
>> + *
>> + * The PPTT structure is an inverted tree, with each node potentially
>> + * holding one or two inverted tree data structures describing
>> + * the caches available at that level. Each cache structure optionally
>> + * contains properties describing the cache at a given level which can be
>> + * used to override hardware probed values.
>> + */
>> +#define pr_fmt(fmt) "ACPI PPTT: " fmt
>> +
>> +#include <linux/acpi.h>
>> +#include <linux/cacheinfo.h>
>> +#include <acpi/processor.h>
>> +
>> +/* total number of attributes checked by the properties code */
>> +#define PPTT_CHECKED_ATTRIBUTES 6
> 
> See comment on this below. If we retain this, move it closer to the usage so
> that it's easier to understand what it actually stands for.

Sure.

> 
>> +
>> +/*
>> + * Given the PPTT table, find and verify that the subtable entry
>> + * is located within the table
>> + */
>> +static struct acpi_subtable_header *fetch_pptt_subtable(
>> +	struct acpi_table_header *table_hdr, u32 pptt_ref)
>> +{
>> +	struct acpi_subtable_header *entry;
>> +
>> +	/* there isn't a subtable at reference 0 */
>> +	if (pptt_ref < sizeof(struct acpi_subtable_header))
>> +		return NULL;
>> +
>> +	if (pptt_ref + sizeof(struct acpi_subtable_header) > table_hdr->length)
>> +		return NULL;
>> +
>> +	entry = ACPI_ADD_PTR(struct acpi_subtable_header, table_hdr, pptt_ref);
>> +
>> +	if (pptt_ref + entry->length > table_hdr->length)
>> +		return NULL;
>> +
>> +	return entry;
>> +}
>> +
>> +static struct acpi_pptt_processor *fetch_pptt_node(
>> +	struct acpi_table_header *table_hdr, u32 pptt_ref)
>> +{
>> +	return (struct acpi_pptt_processor *)fetch_pptt_subtable(table_hdr,
>> +								 pptt_ref);
>> +}
>> +
>> +static struct acpi_pptt_cache *fetch_pptt_cache(
>> +	struct acpi_table_header *table_hdr, u32 pptt_ref)
>> +{
>> +	return (struct acpi_pptt_cache *)fetch_pptt_subtable(table_hdr,
>> +							     pptt_ref);
>> +}
>> +
>> +static struct acpi_subtable_header *acpi_get_pptt_resource(
>> +	struct acpi_table_header *table_hdr,
>> +	struct acpi_pptt_processor *node, int resource)
>> +{
>> +	u32 *ref;
>> +
>> +	if (resource >= node->number_of_priv_resources)
>> +		return NULL;
>> +
>> +	ref = ACPI_ADD_PTR(u32, node, sizeof(struct acpi_pptt_processor));
>> +	ref += resource;
>> +
>> +	return fetch_pptt_subtable(table_hdr, *ref);
>> +}
>> +
>> +/*
>> + * Attempt to find a given cache level, while counting the max number
>> + * of cache levels for the cache node.
>> + *
>> + * Given a pptt resource, verify that it is a cache node, then walk
>> + * down each level of caches, counting how many levels are found
>> + * as well as checking the cache type (icache, dcache, unified). If a
>> + * level & type match, then we set found, and continue the search.
>> + * Once the entire cache branch has been walked return its max
>> + * depth.
>> + */
>> +static int acpi_pptt_walk_cache(struct acpi_table_header *table_hdr,
>> +				int local_level,
>> +				struct acpi_subtable_header *res,
>> +				struct acpi_pptt_cache **found,
>> +				int level, int type)
>> +{
>> +	struct acpi_pptt_cache *cache;
>> +
>> +	if (res->type != ACPI_PPTT_TYPE_CACHE)
>> +		return 0;
>> +
>> +	cache = (struct acpi_pptt_cache *) res;
>> +	while (cache) {
>> +		local_level++;
>> +
>> +		if ((local_level == level) &&
>> +		    (cache->flags & ACPI_PPTT_CACHE_TYPE_VALID) &&
>> +		    ((cache->attributes & ACPI_PPTT_MASK_CACHE_TYPE) == type)) {
>> +			if ((*found != NULL) && (cache != *found))
>> +				pr_err("Found duplicate cache level/type unable to determine uniqueness\n");
>> +
>> +			pr_debug("Found cache @ level %d\n", level);
>> +			*found = cache;
>> +			/*
>> +			 * continue looking at this node's resource list
>> +			 * to verify that we don't find a duplicate
>> +			 * cache node.
>> +			 */
>> +		}
>> +		cache = fetch_pptt_cache(table_hdr, cache->next_level_of_cache);
>> +	}
>> +	return local_level;
>> +}
>> +
>> +/*
>> + * Given a CPU node look for cache levels that exist at this level, and then
>> + * for each cache node, count how many levels exist below (logically above) it.
>> + * If a level and type are specified, and we find that level/type, abort
>> + * processing and return the acpi_pptt_cache structure.
>> + */
>> +static struct acpi_pptt_cache *acpi_find_cache_level(
>> +	struct acpi_table_header *table_hdr,
>> +	struct acpi_pptt_processor *cpu_node,
>> +	int *starting_level, int level, int type)
>> +{
>> +	struct acpi_subtable_header *res;
>> +	int number_of_levels = *starting_level;
>> +	int resource = 0;
>> +	struct acpi_pptt_cache *ret = NULL;
>> +	int local_level;
>> +
>> +	/* walk down from processor node */
>> +	while ((res = acpi_get_pptt_resource(table_hdr, cpu_node, resource))) {
>> +		resource++;
>> +
>> +		local_level = acpi_pptt_walk_cache(table_hdr, *starting_level,
>> +						   res, &ret, level, type);
>> +		/*
>> +		 * we are looking for the max depth. Since its potentially
>> +		 * possible for a given node to have resources with differing
>> +		 * depths verify that the depth we have found is the largest.
>> +		 */
>> +		if (number_of_levels < local_level)
>> +			number_of_levels = local_level;
>> +	}
>> +	if (number_of_levels > *starting_level)
>> +		*starting_level = number_of_levels;
>> +
>> +	return ret;
>> +}
>> +
>> +/*
>> + * Given a processor node containing a processing unit, walk into it and count
>> + * how many levels exist solely for it, and then walk up each level until we hit
>> + * the root node (ignore the package level because it may be possible to have
>> + * caches that exist across packages). Count the number of cache levels that
>> + * exist at each level on the way up.
>> + */
>> +static int acpi_process_node(struct acpi_table_header *table_hdr,
>> +			     struct acpi_pptt_processor *cpu_node)
>> +{
>> +	int total_levels = 0;
>> +
>> +	do {
>> +		acpi_find_cache_level(table_hdr, cpu_node, &total_levels, 0, 0);
>> +		cpu_node = fetch_pptt_node(table_hdr, cpu_node->parent);
>> +	} while (cpu_node);
>> +
>> +	return total_levels;
>> +}
>> +
>> +/*
>> + * Determine if the *node parameter is a leaf node by iterating the
>> + * PPTT table, looking for nodes which reference it.
>> + * Return 0 if we find a node referencing the passed node,
>> + * or 1 if we don't.
>> + */
>> +static int acpi_pptt_leaf_node(struct acpi_table_header *table_hdr,
>> +			       struct acpi_pptt_processor *node)
>> +{
>> +	struct acpi_subtable_header *entry;
>> +	unsigned long table_end;
>> +	u32 node_entry;
>> +	struct acpi_pptt_processor *cpu_node;
>> +
>> +	table_end = (unsigned long)table_hdr + table_hdr->length;
>> +	node_entry = ACPI_PTR_DIFF(node, table_hdr);
>> +	entry = ACPI_ADD_PTR(struct acpi_subtable_header, table_hdr,
>> +			     sizeof(struct acpi_table_pptt));
>> +
>> +	while ((unsigned long)(entry + 1) < table_end) {
> 
> Is entry + 1 check sufficient to access entry of length ?
> Shouldn't that be entry + sizeof(struct acpi_pptt_processor *) so that
> we are sure it's valid entry ?

All we need is the subtable_header size which gives us the type/len. As 
we are just scanning the whole table touching the entry->length and the 
while() terminates if that is > table_end it should be ok. In general 
sizeof(acpi_pptt_processor) isn't right either since the structure only 
covers a larger "header" portion due to attached entries extending 
beyond it.
> 
>> +		cpu_node = (struct acpi_pptt_processor *)entry;
>> +		if ((entry->type == ACPI_PPTT_TYPE_PROCESSOR) &&
>> +		    (cpu_node->parent == node_entry))
>> +			return 0;
>> +		entry = ACPI_ADD_PTR(struct acpi_subtable_header, entry,
>> +				     entry->length);
>> +	}
>> +	return 1;
>> +}
>> +
>> +/*
>> + * Find the subtable entry describing the provided processor.
>> + * This is done by iterating the PPTT table looking for processor nodes
>> + * which have an acpi_processor_id that matches the acpi_cpu_id parameter
>> + * passed into the function. If we find a node that matches this criteria
>> + * we verify that its a leaf node in the topology rather than depending
>> + * on the valid flag, which doesn't need to be set for leaf nodes.
>> + */
>> +static struct acpi_pptt_processor *acpi_find_processor_node(
>> +	struct acpi_table_header *table_hdr,
>> +	u32 acpi_cpu_id)
>> +{
>> +	struct acpi_subtable_header *entry;
>> +	unsigned long table_end;
>> +	struct acpi_pptt_processor *cpu_node;
>> +
>> +	table_end = (unsigned long)table_hdr + table_hdr->length;
>> +	entry = ACPI_ADD_PTR(struct acpi_subtable_header, table_hdr,
>> +			     sizeof(struct acpi_table_pptt));
>> +
>> +	/* find the processor structure associated with this cpuid */
>> +	while ((unsigned long)(entry + 1) < table_end) {
> 
> Same comment as above on entry + 
This one is probably less clear than the one above, because we do access 
a full acpi_pptt_processor sized structure, but only after making sure 
that is actually a processor node. If anything the check should probably 
dereference the len as a second check aka

while ((entry+1 < table_end) && (entry+1->length < table_end))

I think this may have been changed after previous review comments asked 
for the cpu_node assignment earlier and of course moving the leaf_node 
check into the if condition to avoid a bit of extra processing.

> 
>> +		cpu_node = (struct acpi_pptt_processor *)entry;
>> +
>> +		if (entry->length == 0) {
>> +			pr_err("Invalid zero length subtable\n");
>> +			break;
>> +		}
>> +		if ((entry->type == ACPI_PPTT_TYPE_PROCESSOR) &&
>> +		    (acpi_cpu_id == cpu_node->acpi_processor_id) &&
>> +		     acpi_pptt_leaf_node(table_hdr, cpu_node)) {
>> +			return (struct acpi_pptt_processor *)entry;
>> +		}
>> +
>> +		entry = ACPI_ADD_PTR(struct acpi_subtable_header, entry,
>> +				     entry->length);
>> +	}
>> +
>> +	return NULL;
>> +}
>> +
>> +static int acpi_find_cache_levels(struct acpi_table_header *table_hdr,
>> +				  u32 acpi_cpu_id)
>> +{
>> +	int number_of_levels = 0;
>> +	struct acpi_pptt_processor *cpu;
>> +
>> +	cpu = acpi_find_processor_node(table_hdr, acpi_cpu_id);
>> +	if (cpu)
>> +		number_of_levels = acpi_process_node(table_hdr, cpu);
>> +
>> +	return number_of_levels;
>> +}
>> +
>> +/* Convert the linux cache_type to a ACPI PPTT cache type value */
>> +static u8 acpi_cache_type(enum cache_type type)
>> +{
> 
> [nit] Just wondering if we can avoid this with some static mapping:
> 
> static u8 acpi_cache_type[] = {
>          [CACHE_TYPE_NONE] = 0,
>          [CACHE_TYPE_DATA] = ACPI_PPTT_CACHE_TYPE_DATA,
>          [CACHE_TYPE_INST] = ACPI_PPTT_CACHE_TYPE_INSTR,
>          [CACHE_TYPE_UNIFIED] = ACPI_PPTT_CACHE_TYPE_UNIFIED,
> };

Potentially, but the default case below is important and makes it a 
little less brittle because, as the recent DT commit, in your table 
TYPE_NONE actually needs to map to ACPI TYPE_UNIFIED to find the nodes.

Doesn't matter much to me, and I would convert it if the switch() got a 
lot bigger, but right now I tend to think what the code actually would 
look like is a two entry conversion (data/instruction) with a default 
initially set. So a loop for two entries is borderline IMHO.

> 
>> +	switch (type) {
>> +	case CACHE_TYPE_DATA:
>> +		pr_debug("Looking for data cache\n");
>> +		return ACPI_PPTT_CACHE_TYPE_DATA;
>> +	case CACHE_TYPE_INST:
>> +		pr_debug("Looking for instruction cache\n");
>> +		return ACPI_PPTT_CACHE_TYPE_INSTR;
>> +	default:
>> +	case CACHE_TYPE_UNIFIED:
>> +		pr_debug("Looking for unified cache\n");
>> +		/*
>> +		 * It is important that ACPI_PPTT_CACHE_TYPE_UNIFIED
>> +		 * contains the bit pattern that will match both
>> +		 * ACPI unified bit patterns because we use it later
>> +		 * to match both cases.
>> +		 */
>> +		return ACPI_PPTT_CACHE_TYPE_UNIFIED;
>> +	}
>> +}
>> +
>> +/* find the ACPI node describing the cache type/level for the given CPU */
>> +static struct acpi_pptt_cache *acpi_find_cache_node(
>> +	struct acpi_table_header *table_hdr, u32 acpi_cpu_id,
>> +	enum cache_type type, unsigned int level,
>> +	struct acpi_pptt_processor **node)
>> +{
>> +	int total_levels = 0;
>> +	struct acpi_pptt_cache *found = NULL;
>> +	struct acpi_pptt_processor *cpu_node;
>> +	u8 acpi_type = acpi_cache_type(type);
>> +
>> +	pr_debug("Looking for CPU %d's level %d cache type %d\n",
>> +		 acpi_cpu_id, level, acpi_type);
>> +
>> +	cpu_node = acpi_find_processor_node(table_hdr, acpi_cpu_id);
>> +
>> +	while ((cpu_node) && (!found)) {
>> +		found = acpi_find_cache_level(table_hdr, cpu_node,
>> +					      &total_levels, level, acpi_type);
>> +		*node = cpu_node;
>> +		cpu_node = fetch_pptt_node(table_hdr, cpu_node->parent);
>> +	}
>> +
>> +	return found;
>> +}
>> +
>> +/*
>> + * The ACPI spec implies that the fields in the cache structures are used to
>> + * extend and correct the information probed from the hardware. In the case
>> + * of arm64 the CCSIDR probing has been removed because it might be incorrect.
> 
> Though ARM64 is only user now, it may get obsolete, so better to drop that
> comment.

Ok.

> 
>> + */
>> +static void update_cache_properties(struct cacheinfo *this_leaf,
>> +				    struct acpi_pptt_cache *found_cache,
>> +				    struct acpi_pptt_processor *cpu_node)
>> +{
>> +	int valid_flags = 0;
>> +
>> +	if (found_cache->flags & ACPI_PPTT_SIZE_PROPERTY_VALID) {
>> +		this_leaf->size = found_cache->size;
>> +		valid_flags++;
>> +	}
>> +	if (found_cache->flags & ACPI_PPTT_LINE_SIZE_VALID) {
>> +		this_leaf->coherency_line_size = found_cache->line_size;
>> +		valid_flags++;
>> +	}
>> +	if (found_cache->flags & ACPI_PPTT_NUMBER_OF_SETS_VALID) {
>> +		this_leaf->number_of_sets = found_cache->number_of_sets;
>> +		valid_flags++;
>> +	}
>> +	if (found_cache->flags & ACPI_PPTT_ASSOCIATIVITY_VALID) {
>> +		this_leaf->ways_of_associativity = found_cache->associativity;
>> +		valid_flags++;
>> +	}
>> +	if (found_cache->flags & ACPI_PPTT_WRITE_POLICY_VALID) {
>> +		switch (found_cache->attributes & ACPI_PPTT_MASK_WRITE_POLICY) {
>> +		case ACPI_PPTT_CACHE_POLICY_WT:
>> +			this_leaf->attributes = CACHE_WRITE_THROUGH;
>> +			break;
>> +		case ACPI_PPTT_CACHE_POLICY_WB:
>> +			this_leaf->attributes = CACHE_WRITE_BACK;
>> +			break;
>> +		}
>> +		valid_flags++;
>> +	}
>> +	if (found_cache->flags & ACPI_PPTT_ALLOCATION_TYPE_VALID) {
>> +		switch (found_cache->attributes & ACPI_PPTT_MASK_ALLOCATION_TYPE) {
>> +		case ACPI_PPTT_CACHE_READ_ALLOCATE:
>> +			this_leaf->attributes |= CACHE_READ_ALLOCATE;
>> +			break;
>> +		case ACPI_PPTT_CACHE_WRITE_ALLOCATE:
>> +			this_leaf->attributes |= CACHE_WRITE_ALLOCATE;
>> +			break;
>> +		case ACPI_PPTT_CACHE_RW_ALLOCATE:
>> +		case ACPI_PPTT_CACHE_RW_ALLOCATE_ALT:
>> +			this_leaf->attributes |=
>> +				CACHE_READ_ALLOCATE | CACHE_WRITE_ALLOCATE;
>> +			break;
>> +		}
>> +		valid_flags++;
>> +	}
>> +	/*
>> +	 * If all the above flags are valid, and the cache type is NOCACHE
>> +	 * update the cache type as well.
>> +	 */
> 
> I am not sure if it makes sense to mandate at least last 2 (read allocate
> and write policy). They can be optional.

As I mentioned in the previous set, I'm of the opinion that some are 
more useful than others, but to avoid having a discussion about which 
ones, just decided to do them all. After all, its not going to hurt (AFAIK).


If your more _sure_ and no one else has an opinion then i will remove 
those two.


> 
>> +	if ((this_leaf->type == CACHE_TYPE_NOCACHE) &&
>> +	    (valid_flags == PPTT_CHECKED_ATTRIBUTES))
>> +		this_leaf->type = CACHE_TYPE_UNIFIED;
>> +}
>> +
>> +/*
>> + * Update the kernel cache information for each level of cache
>> + * associated with the given acpi cpu.
>> + */
>> +static void cache_setup_acpi_cpu(struct acpi_table_header *table,
>> +				 unsigned int cpu)
>> +{
>> +	struct acpi_pptt_cache *found_cache;
>> +	struct cpu_cacheinfo *this_cpu_ci = get_cpu_cacheinfo(cpu);
>> +	u32 acpi_cpu_id = get_acpi_id_for_cpu(cpu);
>> +	struct cacheinfo *this_leaf;
>> +	unsigned int index = 0;
>> +	struct acpi_pptt_processor *cpu_node = NULL;
>> +
>> +	while (index < get_cpu_cacheinfo(cpu)->num_leaves) {
>> +		this_leaf = this_cpu_ci->info_list + index;
>> +		found_cache = acpi_find_cache_node(table, acpi_cpu_id,
>> +						   this_leaf->type,
>> +						   this_leaf->level,
>> +						   &cpu_node);
>> +		pr_debug("found = %p %p\n", found_cache, cpu_node);
>> +		if (found_cache)
>> +			update_cache_properties(this_leaf,
>> +						found_cache,
>> +						cpu_node);
> 
> [nit] unnecessary line break ?
> 
>> +
>> +		index++;
>> +	}
>> +}
>> +
>> +/**
>> + * acpi_find_last_cache_level() - Determines the number of cache levels for a PE
> 
> [nit] PE ? I think you mean processing element, but that's too ARM ARM thingy
> :), can you s/PE/CPU ?

Yes, I probably slipped up there.

> 
>> + * @cpu: Kernel logical cpu number
>> + *
>> + * Given a logical cpu number, returns the number of levels of cache represented
>> + * in the PPTT. Errors caused by lack of a PPTT table, or otherwise, return 0
>> + * indicating we didn't find any cache levels.
>> + *
>> + * Return: Cache levels visible to this core.
>> + */
>> +int acpi_find_last_cache_level(unsigned int cpu)
>> +{
>> +	u32 acpi_cpu_id;
>> +	struct acpi_table_header *table;
>> +	int number_of_levels = 0;
>> +	acpi_status status;
>> +
>> +	pr_debug("Cache Setup find last level cpu=%d\n", cpu);
>> +
>> +	acpi_cpu_id = get_acpi_id_for_cpu(cpu);
>> +	status = acpi_get_table(ACPI_SIG_PPTT, 0, &table);
>> +	if (ACPI_FAILURE(status)) {
>> +		pr_err_once("No PPTT table found, cache topology may be inaccurate\n");
>> +	} else {
>> +		number_of_levels = acpi_find_cache_levels(table, acpi_cpu_id);
>> +		acpi_put_table(table);
>> +	}
>> +	pr_debug("Cache Setup find last level level=%d\n", number_of_levels);
>> +
>> +	return number_of_levels;
>> +}
>> +
>> +/**
>> + * cache_setup_acpi() - Override CPU cache topology with data from the PPTT
> 
> [nit]			  ^^^^ may be override/setup or just setup ?

I think of it more as "expand upon", or override, but its obviosuly 
creating (setting new) things too.


> 
>> + * @cpu: Kernel logical cpu number
> 
> [nit] kernel is implicit, no ?

Probably...