RE: [RFC 06/20] iommu: Add iommu_device_init[exit]_user_dma interfaces

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Wednesday, September 22, 2021 8:40 PM
> 
> On Wed, Sep 22, 2021 at 01:47:05AM +0000, Tian, Kevin wrote:
> 
> > > IIRC in VFIO the container is the IOAS and when the group goes to
> > > create the device fd it should simply do the
> > > iommu_device_init_user_dma() followed immediately by a call to bind
> > > the container IOAS as your #3.
> >
> > a slight correction.
> >
> > to meet vfio semantics we could do init_user_dma() at group attach
> > time and then call binding to container IOAS when the device fd
> > is created. This is because vfio requires the group in a security context
> > before the device is opened.
> 
> Is it? Until a device FD is opened the group fd is kind of idle, right?

yes, then there is no user-tangible difference between init_user_dma()
at group attach time vs. doing it when opening fd(). But the latter does
require more change than the former, as it also needs the vfio iommu 
driver to provide a .device_attach callback. 

What's in my mind now is to keep existing group attach sequence 
which further calls a group-version init_user_dma(). Then when 
device fd is created, just create a iommu_dev object and switch to
normal fops. 

> 
> > > Ie the basic flow would see the driver core doing some:
> >
> > Just double confirm. Is there concern on having the driver core to
> > call iommu functions?
> 
> It is always an interesting question, but I'd say iommu is
> foundantional to Linux and if it needs driver core help it shouldn't
> be any different from PM, pinctl, or other subsystems that have
> inserted themselves into the driver core.
> 
> Something kind of like the below.
> 
> If I recall, once it is done like this then the entire iommu notifier
> infrastructure can be ripped out which is a lot of code.

thanks for the guidance. will think more along this direction...

> 
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 68ea1f949daa90..e39612c99c6123 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -566,6 +566,10 @@ static int really_probe(struct device *dev, struct
> device_driver *drv)
>                 goto done;
>         }
> 
> +       ret = iommu_set_kernel_ownership(dev);
> +       if (ret)
> +               return ret;
> +
>  re_probe:
>         dev->driver = drv;
> 
> @@ -673,6 +677,7 @@ static int really_probe(struct device *dev, struct
> device_driver *drv)
>                 dev->pm_domain->dismiss(dev);
>         pm_runtime_reinit(dev);
>         dev_pm_set_driver_flags(dev, 0);
> +       iommu_release_kernel_ownership(dev);
>  done:
>         return ret;
>  }
> @@ -1214,6 +1219,7 @@ static void __device_release_driver(struct device
> *dev, struct device *parent)
>                         dev->pm_domain->dismiss(dev);
>                 pm_runtime_reinit(dev);
>                 dev_pm_set_driver_flags(dev, 0);
> +               iommu_release_kernel_ownership(dev);
> 
>                 klist_remove(&dev->p->knode_driver);
>                 device_pm_check_callbacks(dev);