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Subject: Re: [RFC PATCH v2 08/10] vfio/type1: Add domain at(de)taching group
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To:     Lu Baolu <baolu.lu@linux.intel.com>,
        Joerg Roedel <joro@8bytes.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        Kirti Wankhede <kwankhede@nvidia.com>
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        "ashok.raj@intel.com" <ashok.raj@intel.com>,
        "tiwei.bie@intel.com" <tiwei.bie@intel.com>,
        "sanjay.k.kumar@intel.com" <sanjay.k.kumar@intel.com>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "yi.y.sun@intel.com" <yi.y.sun@intel.com>,
        "jacob.jun.pan@intel.com" <jacob.jun.pan@intel.com>,
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 <a97c8f75-ff49-5b87-198f-d3c8950d4e90@linux.intel.com>
From:   Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Message-ID: <04f5dc9d-71b1-37ec-402b-fae5f9e08664@arm.com>
Date:   Wed, 12 Sep 2018 18:54:26 +0100
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On 12/09/2018 06:02, Lu Baolu wrote:
> Hi,
> 
> On 09/11/2018 12:23 AM, Jean-Philippe Brucker wrote:
>> On 30/08/2018 05:09, Lu Baolu wrote:
>>> +static int vfio_detach_aux_domain(struct device *dev, void *data)
>>> +{
>>> +	struct iommu_domain *domain = data;
>>> +
>>> +	vfio_mdev_set_aux_domain(dev, NULL);
>>> +	iommu_detach_device(domain, dev->parent);
>>
>> I think that's only going to work for vt-d, which doesn't use a
>> default_domain. For other drivers, iommu.c ends up calling
>> domain->ops->attach_dev(default_domain, dev) here, which isn't what we want.
> 
> This doesn't break any functionality. It takes effect only if iommu
> hardware supports finer granular translation and advertises it through
> the API.>
>> That was my concern with reusing attach/detach_dev callbacks for PASID
>> management. The attach_dev code of IOMMU drivers already has to deal
>> with toggling between default and unmanaged domain. Dealing with more
>> state transitions in the same path is going to be difficult.
> 
> Let's consider it from the API point of view.
> 
> We have iommu_a(de)ttach_device() APIs to attach or detach a domain
> to/from a device. We should avoid applying a limitation of "these are
> only for single domain case, for multiple domains, use another API".

That's an idealized view of the API, the actual semantics aren't as
simple. For IOMMU drivers that implement IOMMU_DOMAIN_DMA in their
domain_alloc operation (Arm SMMU, AMD IOMMU, ...), attach_dev attaches
an unmanaged domain, detach_dev reattaches the default DMA domain, and
the detach_dev IOMMU op is not called. We can't change that now, so it's
difficult to add more functionality to attach_dev and detach_dev.

Thanks,
Jean