Subject: Re: [PATCH 1/8] iommu: Add I/O ASID allocator

From: Jonathan Cameron <[email-masked]>
Date: Tue, 11 Jun 2019 10:36:25 +0100

On Mon, 10 Jun 2019 19:47:07 +0100
Jean-Philippe Brucker <[email-masked]> wrote:

> 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.
> 
> Signed-off-by: Jean-Philippe Brucker <[email-masked]>
> Signed-off-by: Jacob Pan <[email-masked]>
Hi,

A few trivial comments inline.  May be more because I'm not that familiar
with xa_array than anything else.

Jonathan

> ---
> The most recent discussion on this patch was at:
> https://lkml.kernel.org/lkml/[email-masked]/
> I fixed it up a bit following comments in that series, and removed the
> definitions for the custom allocator for now.
> 
> There also is a new version that includes the custom allocator into this
> patch, but is currently missing the RCU fixes, at:
> https://lore.kernel.org/lkml/[email-masked]/
> ---

...

> +
> +/**
> + * 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. The @private pointer is stored
> + * internally and can be retrieved with ioasid_find().
> + *
> + * Return: the allocated ID on success, or %INVALID_IOASID on failure.
> + */
> +ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t max,
> +		      void *private)
> +{
> +	u32 id = INVALID_IOASID;
> +	struct ioasid_data *data;
> +
> +	data = kzalloc(sizeof(*data), GFP_KERNEL);
> +	if (!data)
> +		return INVALID_IOASID;
> +
> +	data->set = set;
> +	data->private = private;
> +
> +	if (xa_alloc(&ioasid_xa, &id, data, XA_LIMIT(min, max), GFP_KERNEL)) {
> +		pr_err("Failed to alloc ioasid from %d to %d\n", min, max);
> +		goto exit_free;
> +	}
> +	data->id = id;
> +
> +exit_free:

This error flow is perhaps a little more confusing than it needs to be?

My assumption (perhaps wrong) is that we only have an id == INVALID_IOASID
if the xa_alloc fails, and that we will always have such an id value if
it does (I'm not totally sure this second element is true in __xa_alloc).

If I'm missing something perhaps a comment on how else we'd get here.

> +	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;
> +
> +	ioasid_data = xa_erase(&ioasid_xa, ioasid);
> +
> +	kfree_rcu(ioasid_data, rcu);
> +}
> +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.

Perhaps should make it clear that @set can be null.

> + */
> +void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
> +		  bool (*getter)(void *))
> +{
> +	void *priv = NULL;

Set in all paths, so does need to be set here.

> +	struct ioasid_data *ioasid_data;
> +
> +	rcu_read_lock();
> +	ioasid_data = xa_load(&ioasid_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 = rcu_dereference(ioasid_data->private);
> +	if (getter && !getter(priv))
> +		priv = NULL;
> +unlock:
> +	rcu_read_unlock();
> +
> +	return priv;
> +}
> +EXPORT_SYMBOL_GPL(ioasid_find);
> +
> +MODULE_LICENSE("GPL");
...

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