Re: [PATCH v3 1/2] vfio: Replace the DMA unmapping notifier with a callback

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jul 07, 2022 at 03:37:16PM -0600, Alex Williamson wrote:
> On Mon,  4 Jul 2022 21:59:03 -0300
> Jason Gunthorpe <jgg@nvidia.com> wrote:
> > diff --git a/drivers/s390/cio/vfio_ccw_ops.c b/drivers/s390/cio/vfio_ccw_ops.c
> > index b49e2e9db2dc6f..09e0ce7b72324c 100644
> > --- a/drivers/s390/cio/vfio_ccw_ops.c
> > +++ b/drivers/s390/cio/vfio_ccw_ops.c
> > @@ -44,31 +44,19 @@ static int vfio_ccw_mdev_reset(struct vfio_ccw_private *private)
> >  	return ret;
> >  }
> >  
> > -static int vfio_ccw_mdev_notifier(struct notifier_block *nb,
> > -				  unsigned long action,
> > -				  void *data)
> > +static void vfio_ccw_dma_unmap(struct vfio_device *vdev, u64 iova, u64 length)
> >  {
> >  	struct vfio_ccw_private *private =
> > -		container_of(nb, struct vfio_ccw_private, nb);
> > -
> > -	/*
> > -	 * Vendor drivers MUST unpin pages in response to an
> > -	 * invalidation.
> > -	 */
> > -	if (action == VFIO_IOMMU_NOTIFY_DMA_UNMAP) {
> > -		struct vfio_iommu_type1_dma_unmap *unmap = data;
> > -
> > -		if (!cp_iova_pinned(&private->cp, unmap->iova))
> > -			return NOTIFY_OK;
> > +		container_of(vdev, struct vfio_ccw_private, vdev);
> >  
> > -		if (vfio_ccw_mdev_reset(private))
> > -			return NOTIFY_BAD;
> > +	/* Drivers MUST unpin pages in response to an invalidation. */
> > +	if (!cp_iova_pinned(&private->cp, iova))
> > +		return;
> >  
> > -		cp_free(&private->cp);
> > -		return NOTIFY_OK;
> > -	}
> > +	if (vfio_ccw_mdev_reset(private))
> > +		return;
> >  
> > -	return NOTIFY_DONE;
> > +	cp_free(&private->cp);
> >  }
> 
> 
> The cp_free() call is gone here with [1], so I think this function now
> just ends with:
> 
> 	...
> 	vfio_ccw_mdev_reset(private);
> }
> 
> There are also minor contextual differences elsewhere from that series,
> so a quick respin to record the changes on list would be appreciated.
> 
> However the above kind of highlights that NOTIFY_BAD that silently gets
> dropped here.  I realize we weren't testing the return value of the
> notifier call chain and really we didn't intend that notifiers could
> return a failure here, but does this warrant some logging or suggest
> future work to allow a device to go offline here?  Thanks.

It looks like no.

If the FSM trapped in a bad state here, such as
VFIO_CCW_STATE_NOT_OPER, then it means it should have already unpinned
the pages and this is considered a success for this purpose

The return code here exists only to return to userspace so it can
detect during a VFIO_DEVICE_RESET that the device has crashed
irrecoverably.

Thus just continuing to silently ignore it seems like the best thing.

Jason