linux-kernel - Re: [PATCH 1/8] iommu: Add I/O ASID allocator

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Message-ID: <62d1f310-0cba-4d55-0f16-68bba3c64927@arm.com>
Date:   Tue, 11 Jun 2019 15:35:22 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Jonathan Cameron <jonathan.cameron@...wei.com>
Cc:     mark.rutland@....com, devicetree@...r.kernel.org,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, robh+dt@...nel.org,
        robin.murphy@....com, linux-arm-kernel@...ts.infradead.org,
        "jacob.jun.pan@...ux.intel.com" <jacob.jun.pan@...ux.intel.com>
Subject: Re: [PATCH 1/8] iommu: Add I/O ASID allocator

On 11/06/2019 10:36, Jonathan Cameron wrote:
>> +/**
>> + * 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.

Yes we can simplify this:

		return id;
	exit_free:
		kfree(data)
		return INVALID_IOASID;
	}

The XA API doesn't say that @id passed to xa_alloc() won't be modified
in case of error. It's true in the current implementation, but won't
necessarily stay that way. On the other hand I think it's safe to expect
@id to always be set when xa_alloc() succeeds.

>> +/**
>> + * 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.

Indeed. But I'm not sure allowing @set to be NULL is such a good idea,
because the data type associated to an ioasid depends on its set. For
example SVA will put an mm_struct in there, and auxiliary domains use
some structure private to the IOMMU domain.

Jacob, could me make @set mandatory, or do you see a use for a global
search? If @set is NULL, then callers can check if the return pointer is
NULL, but will run into trouble if they try to dereference it.

> 
>> + */
>> +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.

Right

Thanks,
Jean