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Date:   Tue, 18 Jun 2019 17:50:59 +0100
From:   Jonathan Cameron <jonathan.cameron@huawei.com>
To:     Jacob Pan <jacob.jun.pan@linux.intel.com>
CC:     <iommu@lists.linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        David Woodhouse <dwmw2@infradead.org>,
        "Eric Auger" <eric.auger@redhat.com>,
        Alex Williamson <alex.williamson@redhat.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
        "Tian, Kevin" <kevin.tian@intel.com>,
        Raj Ashok <ashok.raj@intel.com>,
        Andriy Shevchenko <andriy.shevchenko@linux.intel.com>
Subject: Re: [PATCH v4 12/22] iommu: Add I/O ASID allocator
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        <1560087862-57608-13-git-send-email-jacob.jun.pan@linux.intel.com>
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On Sun, 9 Jun 2019 06:44:12 -0700
Jacob Pan <jacob.jun.pan@linux.intel.com> wrote:

> From: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> 
> Some devices might support multiple DMA address spaces, in particular
> those that have the PCI PASID feature. PASID (Process Address Space ID)
> allows to share process address spaces with devices (SVA), partition a
> device into VM-assignable entities (VFIO mdev) or simply provide
> multiple DMA address space to kernel drivers. Add a global PASID
> allocator usable by different drivers at the same time. Name it I/O ASID
> to avoid confusion with ASIDs allocated by arch code, which are usually
> a separate ID space.
> 
> The IOASID space is global. Each device can have its own PASID space,
> but by convention the IOMMU ended up having a global PASID space, so
> that with SVA, each mm_struct is associated to a single PASID.
> 
> The allocator is primarily used by IOMMU subsystem but in rare occasions
> drivers would like to allocate PASIDs for devices that aren't managed by
> an IOMMU, using the same ID space as IOMMU.
> 
> There are two types of allocators:
> 1. default allocator - Always available, uses an XArray to track
> 2. custom allocators - Can be registered at runtime, take precedence
> over the default allocator.
> 
> Custom allocators have these attributes:
> - provides platform specific alloc()/free() functions with private data.
> - allocation results lookup are not provided by the allocator, lookup
>   request must be done by the IOASID framework by its own XArray.
> - allocators can be unregistered at runtime, either fallback to the next
>   custom allocator or to the default allocator.

What is the usecase for having a 'stack' of custom allocators?

> - custom allocators can share the same set of alloc()/free() helpers, in
>   this case they also share the same IOASID space, thus the same XArray.
> - switching between allocators requires all outstanding IOASIDs to be
>   freed unless the two allocators share the same alloc()/free() helpers.
In general this approach has a lot of features where the justification is
missing from this particular patch.  It may be useful to add some
more background to this intro?

> 
> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
> Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> Link: https://lkml.org/lkml/2019/4/26/462

Various comments inline.  Given the several cups of coffee this took to
review I may well have misunderstood everything ;)

Jonathan
> ---
>  drivers/iommu/Kconfig  |   8 +
>  drivers/iommu/Makefile |   1 +
>  drivers/iommu/ioasid.c | 427 +++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/ioasid.h |  74 +++++++++
>  4 files changed, 510 insertions(+)
>  create mode 100644 drivers/iommu/ioasid.c
>  create mode 100644 include/linux/ioasid.h
> 
> diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> index 83664db..c40c4b5 100644
> --- a/drivers/iommu/Kconfig
> +++ b/drivers/iommu/Kconfig
> @@ -3,6 +3,13 @@
>  config IOMMU_IOVA
>  	tristate
>  
> +# The IOASID allocator may also be used by non-IOMMU_API users
> +config IOASID
> +	tristate
> +	help
> +	  Enable the I/O Address Space ID allocator. A single ID space shared
> +	  between different users.
> +
>  # IOMMU_API always gets selected by whoever wants it.
>  config IOMMU_API
>  	bool
> @@ -207,6 +214,7 @@ config INTEL_IOMMU_SVM
>  	depends on INTEL_IOMMU && X86
>  	select PCI_PASID
>  	select MMU_NOTIFIER
> +	select IOASID
>  	help
>  	  Shared Virtual Memory (SVM) provides a facility for devices
>  	  to access DMA resources through process address space by
> diff --git a/drivers/iommu/Makefile b/drivers/iommu/Makefile
> index 8c71a15..0efac6f 100644
> --- a/drivers/iommu/Makefile
> +++ b/drivers/iommu/Makefile
> @@ -7,6 +7,7 @@ obj-$(CONFIG_IOMMU_DMA) += dma-iommu.o
>  obj-$(CONFIG_IOMMU_IO_PGTABLE) += io-pgtable.o
>  obj-$(CONFIG_IOMMU_IO_PGTABLE_ARMV7S) += io-pgtable-arm-v7s.o
>  obj-$(CONFIG_IOMMU_IO_PGTABLE_LPAE) += io-pgtable-arm.o
> +obj-$(CONFIG_IOASID) += ioasid.o
>  obj-$(CONFIG_IOMMU_IOVA) += iova.o
>  obj-$(CONFIG_OF_IOMMU)	+= of_iommu.o
>  obj-$(CONFIG_MSM_IOMMU) += msm_iommu.o
> diff --git a/drivers/iommu/ioasid.c b/drivers/iommu/ioasid.c
> new file mode 100644
> index 0000000..0919b70
> --- /dev/null
> +++ b/drivers/iommu/ioasid.c
> @@ -0,0 +1,427 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * I/O Address Space ID allocator. There is one global IOASID space, split into
> + * subsets. Users create a subset with DECLARE_IOASID_SET, then allocate and
> + * free IOASIDs with ioasid_alloc and ioasid_free.
> + */
> +#define pr_fmt(fmt)	KBUILD_MODNAME ": " fmt
> +
> +#include <linux/xarray.h>
> +#include <linux/ioasid.h>
> +#include <linux/slab.h>
> +#include <linux/module.h>
> +#include <linux/spinlock.h>
> +
> +struct ioasid_data {
> +	ioasid_t id;
> +	struct ioasid_set *set;
> +	void *private;
> +	struct rcu_head rcu;
> +};
> +
> +/*
> + * struct ioasid_allocator_data - Internal data structure to hold information
> + * about an allocator. There are two types of allocators:
> + *
> + * - Default allocator always has its own XArray to track the IOASIDs allocated.
> + * - Custom allocators may share allocation helpers with different private data.
> + *   Custom allocators share the same helper functions also share the same
> + *   XArray.
> + * Rules:
> + * 1. Default allocator is always available, not dynamically registered. This is
> + *    to prevent race conditions with early boot code that want to register
> + *    custom allocators or allocate IOASIDs.
> + * 2. Custom allocators take precedence over the default allocator.
> + * 3. When all custom allocators sharing the same helper functions are
> + *    unregistered (e.g. due to hotplug), all outstanding IOASIDs must be
> + *    freed.
> + * 4. When switching between custom allocators sharing the same helper
> + *    functions, outstanding IOASIDs are preserved.
> + * 5. When switching between custom allocator and default allocator, all IOASIDs
> + *    must be freed to ensure unadulterated space for the new allocator.
> + *
> + * @ops:	allocator helper functions and its data
> + * @list:	registered custom allocators
> + * @slist:	allocators share the same ops but different data
> + * @flags:	attributes of the allocator
> + * @users	number of allocators sharing the same ops and XArray
> + * @xa		xarray holds the IOASID space
Doc ordering should match the struct.

> + */
> +struct ioasid_allocator_data {
> +	struct ioasid_allocator_ops *ops;
> +	struct list_head list;
> +	struct list_head slist;
> +#define IOASID_ALLOCATOR_CUSTOM BIT(0) /* Needs framework to track results */
> +	unsigned long flags;
> +	struct xarray xa;
> +	refcount_t users;
> +};
> +
> +static DEFINE_MUTEX(ioasid_allocator_lock);
> +static LIST_HEAD(allocators_list);
> +static ioasid_t default_alloc(ioasid_t min, ioasid_t max, void *opaque);
> +static void default_free(ioasid_t ioasid, void *opaque);
> +
> +static struct ioasid_allocator_ops default_ops = {
> +	.alloc = default_alloc,
> +	.free = default_free

Pure churn prevention but doesn't seem implausible that we'll end up with
more ops at somepoint, so add the commas on last elements?

> +};
> +
> +static struct ioasid_allocator_data default_allocator = {
> +	.ops = &default_ops,
> +	.flags = 0,
> +	.xa = XARRAY_INIT(ioasid_xa, XA_FLAGS_ALLOC)
> +};
> +
> +static struct ioasid_allocator_data *active_allocator = &default_allocator;
> +
> +static ioasid_t default_alloc(ioasid_t min, ioasid_t max, void *opaque)
> +{
Why not reorder to avoid having the forward defs above?  It's only moving things
a few lines in this case so I'm not seeing a strong readability argument.

> +	ioasid_t id;
> +
> +	if (xa_alloc(&default_allocator.xa, &id, opaque, XA_LIMIT(min, max), GFP_KERNEL)) {
> +		pr_err("Failed to alloc ioasid from %d to %d\n", min, max);

It seems xa_alloc can return a few different error values. Perhaps worth printing
out what we got as might help in debugging?

> +		return INVALID_IOASID;
> +	}
> +
> +	return id;
> +}
> +
> +void default_free(ioasid_t ioasid, void *opaque)
> +{
> +	struct ioasid_data *ioasid_data;
> +
> +	ioasid_data = xa_erase(&default_allocator.xa, ioasid);
> +	kfree_rcu(ioasid_data, rcu);
> +}
> +
> +/* Allocate and initialize a new custom allocator with its helper functions */
> +static inline struct ioasid_allocator_data *ioasid_alloc_allocator(struct ioasid_allocator_ops *ops)

Why inline?

> +{
> +	struct ioasid_allocator_data *ia_data;
> +
> +	ia_data = kzalloc(sizeof(*ia_data), GFP_KERNEL);
> +	if (!ia_data)
> +		return NULL;
> +
> +	xa_init_flags(&ia_data->xa, XA_FLAGS_ALLOC);
> +	INIT_LIST_HEAD(&ia_data->slist);
> +	ia_data->flags |= IOASID_ALLOCATOR_CUSTOM;
> +	ia_data->ops = ops;
> +
> +	/* For tracking custom allocators that share the same ops */
> +	list_add_tail(&ops->list, &ia_data->slist);
> +	refcount_set(&ia_data->users, 1);
> +
> +	return ia_data;
> +}
> +
> +static inline bool use_same_ops(struct ioasid_allocator_ops *a, struct ioasid_allocator_ops *b)
> +{
> +	return (a->free == b->free) && (a->alloc == b->alloc);
> +}
> +
> +/**
> + * ioasid_register_allocator - register a custom allocator
> + * @ops: the custom allocator ops to be registered
> + *
> + * Custom allocators take precedence over the default xarray based allocator.
> + * Private data associated with the IOASID allocated by the custom allocators
> + * are managed by IOASID framework similar to data stored in xa by default
> + * allocator.
> + *
> + * There can be multiple allocators registered but only one is active. In case
> + * of runtime removal of a custom allocator, the next one is activated based
> + * on the registration ordering.
> + *
> + * Multiple allocators can share the same alloc() function, in this case the
> + * IOASID space is shared.
> + *
Nitpick: This extra blank line seems inconsistent.

> + */
> +int ioasid_register_allocator(struct ioasid_allocator_ops *ops)
> +{
> +	struct ioasid_allocator_data *ia_data;
> +	struct ioasid_allocator_data *pallocator;
> +	int ret = 0;
> +
> +	mutex_lock(&ioasid_allocator_lock);
> +
> +	ia_data = ioasid_alloc_allocator(ops);
> +	if (!ia_data) {
> +		ret = -ENOMEM;
> +		goto out_unlock;
> +	}
> +
> +	/*
> +	 * No particular preference, we activate the first one and keep
> +	 * the later registered allocators in a list in case the first one gets
> +	 * removed due to hotplug.
> +	 */
> +	if (list_empty(&allocators_list)) {
> +		WARN_ON(active_allocator != &default_allocator);
> +		/* Use this new allocator if default is not active */
> +		if (xa_empty(&active_allocator->xa)) {
> +			active_allocator = ia_data;
> +			list_add_tail(&ia_data->list, &allocators_list);
> +			goto out_unlock;
> +		}
> +		pr_warn("Default allocator active with outstanding IOASID\n");
> +		ret = -EAGAIN;
> +		goto out_free;
> +	}
> +
> +	/* Check if the allocator is already registered */
> +	list_for_each_entry(pallocator, &allocators_list, list) {
> +		if (pallocator->ops == ops) {
> +			pr_err("IOASID allocator already registered\n");
> +			ret = -EEXIST;
> +			goto out_free;
> +		} else if (use_same_ops(pallocator->ops, ops)) {
> +			/*
> +			 * If the new allocator shares the same ops,
> +			 * then they will share the same IOASID space.
> +			 * We should put them under the same xarray.
> +			 */
> +			list_add_tail(&ops->list, &pallocator->slist);
> +			refcount_inc(&pallocator->users);
> +			pr_info("New IOASID allocator ops shared %u times\n",
> +				refcount_read(&pallocator->users));
> +			goto out_free;
> +		}
> +	}
> +	list_add_tail(&ia_data->list, &allocators_list);
> +
> +	goto out_unlock;
I commented on this in Jean-Phillipe's set as well I think.
Cleaner to just have a separate unlock copy here and return.

> +
> +out_free:
> +	kfree(ia_data);
> +out_unlock:
> +	mutex_unlock(&ioasid_allocator_lock);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_register_allocator);
> +
> +static inline bool has_shared_ops(struct ioasid_allocator_data *allocator)
> +{
> +	return refcount_read(&allocator->users) > 1;
> +}
> +
> +/**
> + * ioasid_unregister_allocator - Remove a custom IOASID allocator ops
> + * @ops: the custom allocator to be removed
> + *
> + * Remove an allocator from the list, activate the next allocator in
> + * the order it was registered. Or revert to default allocator if all
> + * custom allocators are unregistered without outstanding IOASIDs.
> + */
> +void ioasid_unregister_allocator(struct ioasid_allocator_ops *ops)
> +{
> +	struct ioasid_allocator_data *pallocator;
> +	struct ioasid_allocator_ops *sops;
> +
> +	mutex_lock(&ioasid_allocator_lock);
> +	if (list_empty(&allocators_list)) {
> +		pr_warn("No custom IOASID allocators active!\n");
> +		goto exit_unlock;
> +	}
> +
> +	list_for_each_entry(pallocator, &allocators_list, list) {
> +		if (use_same_ops(pallocator->ops, ops)) {
Could reduce the depth with.

		if (!use_same_ops(pallocator->ops, ops))
			continue;

Trade off between having the unusual flow that will resort and the
code going off the side of your screen (if you use a very small
screen ;)

> +			if (refcount_read(&pallocator->users) == 1) {
> +				/* No shared helper functions */
> +				list_del(&pallocator->list);
> +				/*
> +				 * All IOASIDs should have been freed before
> +				 * the last allocator that shares the same ops
> +				 * is unregistered.
> +				 */
> +				WARN_ON(!xa_empty(&pallocator->xa));
> +				kfree(pallocator);
> +				if (list_empty(&allocators_list)) {
> +					pr_info("No custom IOASID allocators, switch to default.\n");
> +					active_allocator = &default_allocator;
> +				} else if (pallocator == active_allocator) {
> +					active_allocator = list_entry(&allocators_list, struct ioasid_allocator_data, list);

What is the intent here?  You seem to be calling the list entry dereference on
the list head itself.  That will get you something fairly random on the stack 
I think.

list_first_entry maybe?


> +					pr_info("IOASID allocator changed");
> +				}
> +				break;
> +			}
> +			/*
> +			 * Find the matching shared ops to delete,
> +			 * but keep outstanding IOASIDs
> +			 */
> +			list_for_each_entry(sops, &pallocator->slist, list) {
> +				if (sops == ops) {
> +					list_del(&ops->list);
> +					if (refcount_dec_and_test(&pallocator->users))
> +						pr_err("no shared ops\n");
> +					break;
> +				}
> +			}
> +			break;
> +		}
> +	}
> +
> +exit_unlock:
> +	mutex_unlock(&ioasid_allocator_lock);
> +}
> +EXPORT_SYMBOL_GPL(ioasid_unregister_allocator);
> +
> +/**
> + * ioasid_set_data - Set private data for an allocated ioasid
> + * @ioasid: the ID to set data
> + * @data:   the private data
> + *
> + * For IOASID that is already allocated, private data can be set
> + * via this API. Future lookup can be done via ioasid_find.
> + */
> +int ioasid_set_data(ioasid_t ioasid, void *data)
> +{
> +	struct ioasid_data *ioasid_data;
> +	int ret = 0;
> +
> +	mutex_lock(&ioasid_allocator_lock);
> +	ioasid_data = xa_load(&active_allocator->xa, ioasid);
> +	if (ioasid_data)
> +		ioasid_data->private = data;
> +	else
> +		ret = -ENOENT;
> +	mutex_unlock(&ioasid_allocator_lock);
> +
> +	/* getter may use the private data */
> +	synchronize_rcu();
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_set_data);
> +
> +/**
> + * ioasid_alloc - Allocate an IOASID
> + * @set: the IOASID set
> + * @min: the minimum ID (inclusive)
> + * @max: the maximum ID (inclusive)
> + * @private: data private to the caller
> + *
> + * Allocate an ID between @min and @max. Return the allocated ID on success,
> + * or INVALID_IOASID on failure. The @private pointer is stored internally
> + * and can be retrieved with ioasid_find().
> + */
> +ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t max,
> +		      void *private)
> +{
> +	ioasid_t id = INVALID_IOASID;
> +	struct ioasid_data *data;
> +
> +	data = kzalloc(sizeof(*data), GFP_KERNEL);
> +	if (!data)
> +		return INVALID_IOASID;
> +
> +	data->set = set;
> +	data->private = private;
> +
> +	mutex_lock(&ioasid_allocator_lock);
> +
> +	id = active_allocator->ops->alloc(min, max, data);
> +	if (id == INVALID_IOASID) {
> +		pr_err("Failed ASID allocation %lu\n", active_allocator->flags);
> +			mutex_unlock(&ioasid_allocator_lock);
> +			goto exit_free;
> +	}
> +	if (active_allocator->flags & IOASID_ALLOCATOR_CUSTOM) {
> +		/* Custom allocator needs framework to store and track allocation results */
> +		min = id;
> +		max = id + 1;

Looking at xa_limit definition these are inclusive limits so why do we let it take
either value?

> +
> +		if (xa_alloc(&active_allocator->xa, &id, data, XA_LIMIT(min, max), GFP_KERNEL)) {
> +			pr_err("Failed to alloc ioasid from %d to %d\n", min, max);
> +			active_allocator->ops->free(id, NULL);
> +			goto exit_free;
> +		}
> +	}
> +	data->id = id;
> +
> +	mutex_unlock(&ioasid_allocator_lock);
> +
> +exit_free:
As with the version of the patch without custom allocator, feels like we can simplify this
section as we can't get here with id == INVALID_IOASID unless we went to the exit_free.

If we did take the exit_free it is always INVALID_IOASID ( I think, but haven't checked...)

So just have a return from the good path and always run
this block i the bad one.
 
> +	if (id == INVALID_IOASID) {
> +		kfree(data);
> +		return INVALID_IOASID;
> +	}
> +	return id;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_alloc);
> +
> +/**
> + * ioasid_free - Free an IOASID
> + * @ioasid: the ID to remove
> + */
> +void ioasid_free(ioasid_t ioasid)
> +{
> +	struct ioasid_data *ioasid_data;
> +
> +	mutex_lock(&ioasid_allocator_lock);
> +
> +	ioasid_data = xa_load(&active_allocator->xa, ioasid);
> +	if (!ioasid_data) {
> +		pr_err("Trying to free unknown IOASID %u\n", ioasid);
> +		goto exit_unlock;
> +	}
> +
> +	active_allocator->ops->free(ioasid, active_allocator->ops->pdata);
> +	/* Custom allocator needs additional steps to free the xa */
xa element perhaps?

> +	if (active_allocator->flags & IOASID_ALLOCATOR_CUSTOM) {
> +		ioasid_data = xa_erase(&active_allocator->xa, ioasid);
> +		kfree_rcu(ioasid_data, rcu);
> +	}
> +
> +exit_unlock:
> +	mutex_unlock(&ioasid_allocator_lock);
> +}
> +EXPORT_SYMBOL_GPL(ioasid_free);
> +
> +/**
> + * ioasid_find - Find IOASID data
> + * @set: the IOASID set
> + * @ioasid: the IOASID to find
> + * @getter: function to call on the found object
> + *
> + * The optional getter function allows to take a reference to the found object
> + * under the rcu lock. The function can also check if the object is still valid:
> + * if @getter returns false, then the object is invalid and NULL is returned.
> + *
> + * If the IOASID has been allocated for this set, return the private pointer
> + * passed to ioasid_alloc. Private data can be NULL if not set. Return an error
> + * if the IOASID is not found or does not belong to the set.
> + */
> +void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
> +		  bool (*getter)(void *))
> +{
> +	void *priv = NULL;

Always set so no need to initialize.

> +	struct ioasid_data *ioasid_data;
> +
> +	rcu_read_lock();
> +	ioasid_data = xa_load(&active_allocator->xa, ioasid);
> +	if (!ioasid_data) {
> +		priv = ERR_PTR(-ENOENT);
> +		goto unlock;
> +	}
> +	if (set && ioasid_data->set != set) {
> +		/* data found but does not belong to the set */
> +		priv = ERR_PTR(-EACCES);
> +		goto unlock;
> +	}
> +	/* Now IOASID and its set is verified, we can return the private data */
> +	priv = ioasid_data->private;
> +	if (getter && !getter(priv))
> +		priv = NULL;
> +unlock:
> +	rcu_read_unlock();
> +
> +	return priv;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_find);
> +
> +MODULE_AUTHOR("Jean-Philippe Brucker <jean-philippe.brucker@arm.com>");
> +MODULE_AUTHOR("Jacob Pan <jacob.jun.pan@linux.intel.com>");
> +MODULE_DESCRIPTION("IO Address Space ID (IOASID) allocator");
> +MODULE_LICENSE("GPL");
> diff --git a/include/linux/ioasid.h b/include/linux/ioasid.h
> new file mode 100644
> index 0000000..8c8d1c5
> --- /dev/null
> +++ b/include/linux/ioasid.h
> @@ -0,0 +1,74 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __LINUX_IOASID_H
> +#define __LINUX_IOASID_H
> +
> +#define INVALID_IOASID ((ioasid_t)-1)
> +typedef unsigned int ioasid_t;
> +typedef ioasid_t (*ioasid_alloc_fn_t)(ioasid_t min, ioasid_t max, void *data);
> +typedef void (*ioasid_free_fn_t)(ioasid_t ioasid, void *data);
> +
> +struct ioasid_set {
> +	int dummy;
> +};
> +
> +/**
> + * struct ioasid_allocator_ops - IOASID allocator helper functions and data
> + *
> + * @alloc:	helper function to allocate IOASID
> + * @free:	helper function to free IOASID
> + * @list:	for tracking ops that share helper functions but not data
> + * @pdata:	data belong to the allocator, provided when calling alloc()
and free()

Seems odd to call out one and not the other.
I'm not actually sure it's true though as I can't see pdata being provided
to the alloc call.

> + */
> +struct ioasid_allocator_ops {
> +	ioasid_alloc_fn_t alloc;
> +	ioasid_free_fn_t free;
> +	struct list_head list;
> +	void *pdata;
> +};
> +
> +#define DECLARE_IOASID_SET(name) struct ioasid_set name = { 0 }

I was a little curious on how this would actually be used.
Even more curious is that it isn't used that I can find.

> +
> +#if IS_ENABLED(CONFIG_IOASID)
> +ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t max,
> +		      void *private);
> +void ioasid_free(ioasid_t ioasid);
> +
> +void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
> +			bool (*getter)(void *));
> +int ioasid_register_allocator(struct ioasid_allocator_ops *allocator);
> +void ioasid_unregister_allocator(struct ioasid_allocator_ops *allocator);
> +int ioasid_set_data(ioasid_t ioasid, void *data);
> +
> +#else /* !CONFIG_IOASID */
> +static inline ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min,
> +				    ioasid_t max, void *private)
> +{
> +	return INVALID_IOASID;
> +}
> +
> +static inline void ioasid_free(ioasid_t ioasid)
> +{
> +}
> +
> +static inline void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
> +				bool (*getter)(void *))
> +{
> +	return NULL;
> +}
> +
> +static inline int ioasid_register_allocator(struct ioasid_allocator_ops *allocator)
> +{
> +	return -ENODEV;
> +}
> +
> +static inline void ioasid_unregister_allocator(struct ioasid_allocator_ops *allocator)
> +{
> +}
> +
> +static inline int ioasid_set_data(ioasid_t ioasid, void *data)
> +{
> +	return -ENODEV;
> +}
> +
> +#endif /* CONFIG_IOASID */
> +#endif /* __LINUX_IOASID_H */