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Subject: Re: [RFC PATCH v2 08/10] vfio/type1: Add domain at(de)taching group
 helpers
To:     Jean-Philippe Brucker <jean-philippe.brucker@arm.com>,
        "Tian, Kevin" <kevin.tian@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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From:   Lu Baolu <baolu.lu@linux.intel.com>
Message-ID: <1b4c12c3-1dc2-d9ba-9ba4-5220db3e2302@linux.intel.com>
Date:   Wed, 26 Sep 2018 10:11:03 +0800
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Hi,

On 09/26/2018 01:55 AM, Jean-Philippe Brucker wrote:
> On 19/09/2018 00:26, Tian, Kevin wrote:
>>> If the core actually calls it, we can implement detach_dev :) The
>>> problem is that the core never calls detach_dev when default_domain is
>>> present (affects any IOMMU driver that relies on default_domain,
>>> including AMD), and even in case 4) the default_domain is present for
>>> the parent device
>>
>> Then can we change that core logic so detach_dev is invoked in all
>> cases? yes there will be some changes in vendor drivers, but I expect
>> this change trivial (especially considering the gain in IOMMU API
>> simplicity side as described below).
> 
> Thinking more about this, there might be a way that doesn't require too
> much rewriting of IOMMU drivers. When VFIO calls iommu_detach_group to
> detach a vfio-pci device from its domain, it could do detach_dev(dev,
> old_domain) followed by attach_dev(dev, default_domain) instead of only
> attach_dev(dev, default_domain), so:
> 
> __iommu_attach_group(grp, unmanaged_domain)
>    ops->attach_dev(dev, unmanaged_domain)
>      -> IOMMU driver detaches from default_domain only if the dev isn't
>         in auxiliary mode
> 
> __iommu_detach_group(grp, unmanaged_domain)
>    if (ops->detach_dev)
>        ops->detach_dev(dev, unmanaged_domain)
>    ops->attach_dev(dev, default_domain)
>      -> IOMMU driver ignores this if still attached to default_domain
> 

This looks good to me.

Just a minor change to make it more readable.

__iommu_detach_group(grp, unmanaged_domain)
     if (ops->detach_dev)
         ops->detach_dev(dev, unmanaged_domain)
     if (default_domain)
         ops->attach_dev(dev, default_domain)

Best regards,
Lu Baolu

> It's not trivial: since half the IOMMU drivers seem to be using default
> domain now, their detach_dev callback might have been dead code for a
> while (I can't find a path where it still gets called), so re-enabling
> it requires some attention. However, since most of them won't care about
> aux domains, maybe we could just remove their detach_dev pointer.
> 
> The resulting logic of attach/detach_dev in IOMMU drivers that do use
> aux domains still seems unnecessarily convoluted. It could as well be
> done with separate attach_aux/detach_aux functions. If you want to keep
> it that way though, I think the above could work for us and I'll try to
> write an RFC.
> 
> Thanks,
> Jean
>