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

[Lu Baolu]

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