[PATCH 0/2] iommu: Remove iommu_sva_ops::mm_exit()

[Jacob Pan]

On Thu, 9 Apr 2020 09:08:21 -0300
Jason Gunthorpe <jgg at ziepe.ca> wrote:

> On Wed, Apr 08, 2020 at 04:48:02PM -0700, Jacob Pan wrote:
> > > Yes, this is the proper way, when the DMA is stopped and no use
> > > of the PASID remains then you can drop the mmu notifier and
> > > release the PASID entirely. If that is linked to the lifetime of
> > > the FD then forget completely about the mm_struct lifetime, it
> > > doesn't matter.. 
> > Got everything above, thanks a lot.
> > 
> > If everything is in order with the FD close. Why do we need to 
> > "ask IOMMU drivers to silently abort DMA and Page Requests in the
> > meantime." in mm_exit notifier? This will be done orderly in unbind
> > anyway.  
> 
> I thought this is exactly what Jean-Phillippe is removing here, it is
> a bad idea for the reasons he explained.
> 
I think this patch only removed driver side callbacks, i.e. to stop
DMA. But not removing IOMMU side of stop DMA, PRS.

Before uacce, (universal accelerator framework), sva bind/unbind is not
guaranteed at open/close FD time. Therefore, mmu notifier is needed if
mmexit comes without unbind.

> > > > Enforcing unbind upon FD close might be a precarious path,
> > > > perhaps that is why we have to deal with out of order
> > > > situation?    
> > > 
> > > How so? You just put it in the FD release function :)  
> > 
> > I was thinking some driver may choose to defer unbind in some
> > workqueue etc.  
> 
> Doesn't really change anything, the lifetime of the PASID wouuld be
> the lifetime of the notifier and the bind, and DMA can't continue
> without the notifier registered.
> 
True, it is just better not to defer. Otherwise, the window of
suppressing error gets longer.