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

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Date:   Tue, 18 Jun 2019 15:22:20 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Jonathan Cameron <jonathan.cameron@...wei.com>,
        Mark Rutland <Mark.Rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Will Deacon <Will.Deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        Robin Murphy <Robin.Murphy@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/8] iommu: Add I/O ASID allocator

On 11/06/2019 19:13, Jacob Pan wrote:
>>>> +/**
>>>> + * 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.
>>
> I am not sure we need to count on @set to decipher data type. Whoever
> does the allocation and owns the IOASID should knows its own data type.
> My thought was that @set is only used to group IDs, permission check
> etc.
> 
>> 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.
>>
> A global search use case can be for PRQ. IOMMU driver gets a IOASID
> (first interrupt then retrieve from a queue), it has no idea which
> @set it belongs to. But the data types are the same for all IOASIDs
> used by the IOMMU.

They aren't when we use a generic SVA handler. Following a call to
iommu_sva_bind_device(), iommu-sva.c allocates an IOASID and store an
mm_struct. If auxiliary domains are also enabled for the device,
following a call to iommu_aux_attach_device() the IOMMU driver allocates
an IOASID and stores some private object.

Now for example the IOMMU driver receives a PPR and calls ioasid_find()
with @set = NULL. ioasid_find() may return either an mm_struct or a
private object, and the driver cannot know which it is so the returned
value is unusable.

Thanks,
Jean