Re: [RFC PATCH v2 08/10] vfio/type1: Add domain at(de)taching group helpers

[Jean-Philippe Brucker]

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

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