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

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Monday, September 27, 2021 7:54 PM
> 
> On Mon, Sep 27, 2021 at 09:42:58AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Sent: Wednesday, September 22, 2021 8:40 PM
> > >
> > > > > 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.
> >
> > Currently vfio is the only user of this notifier mechanism. Now
> > three events are handled in vfio_iommu_group_notifier():
> >
> > NOTIFY_ADD_DEVICE: this is basically for some sanity check. suppose
> > not required once we handle it cleanly in the iommu/driver core.
> >
> > NOTIFY_BOUND_DRIVER: the BUG_ON() logic to be fixed by this change.
> >
> > NOTIFY_UNBOUND_DRIVER: still needs some thoughts. Based on
> > the comments the group->unbound_list is used to avoid breaking
> 
> I have a patch series to delete the unbound_list, the scenario you
> describe is handled by the device_lock()

that's great!

> 
> > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > index 68ea1f9..826a651 100644
> > +++ b/drivers/base/dd.c
> > @@ -566,6 +566,10 @@ static int really_probe(struct device *dev, struct
> device_driver *drv)
> >  		goto done;
> >  	}
> >
> > +	ret = iommu_device_set_dma_hint(dev, drv->dma_hint);
> > +	if (ret)
> > +		return ret;
> 
> I think for such a narrow usage you should not change the struct
> device_driver. Just have pci_stub call a function to flip back to user
> mode.

Here we want to ensure that kernel dma should be blocked
if the group is already marked for user-dma. If we just blindly
do it for any driver at this point (as you commented earlier):

+       ret = iommu_set_kernel_ownership(dev);
+       if (ret)
+               return ret;

how would pci-stub reach its function to indicate that it doesn't 
do dma and flip back?

Do you envision a simpler policy that no driver can be bound
to the group if it's already set for user-dma? what about vfio-pci
itself?

> 
> > +static int iommu_dev_viable(struct device *dev, void *data)
> > +{
> > +	enum dma_hint hint = *data;
> > +	struct device_driver *drv = READ_ONCE(dev->driver);
> 
> Especially since this isn't locked properly or safe.

I have the same worry when copying from vfio. Not sure how
vfio gets safe with this approach...

Thanks
Kevin