Re: [PATCH v1 5/8] iommufd: Add replace support in iommufd_access_set_ioas()

[Jason Gunthorpe]

On Fri, Feb 03, 2023 at 10:10:45AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > Sent: Thursday, February 2, 2023 3:05 PM
> > 
> > Support an access->ioas replacement in iommufd_access_set_ioas(), which
> > sets the access->ioas to NULL provisionally so that any further incoming
> > iommufd_access_pin_pages() callback can be blocked.
> > 
> > Then, call access->ops->unmap() to clean up the entire iopt. To allow an
> > iommufd_access_unpin_pages() callback to happen via this unmap() call,
> > add an ioas_unpin pointer so the unpin routine won't be affected by the
> > "access->ioas = NULL" trick above.
> > 
> > Also, a vdev without an ops->dma_unmap implementation cannot replace its
> > access->ioas pointer. So add an iommufd_access_ioas_is_attached() helper
> > to sanity that.
> > 
> 
> Presumably a driver which doesn't implement ops->dma_unmap shouldn't
> be allowed to do pin/unpin. But it could use vfio_dma_rw() to access an
> iova range. In the latter case I don't see why replace cannot work.
> 
> Probably what's required here is to deny !ops->dma_unmap in
> vfio_pin/unpin_pages then making here replace always allowed?

That makes sense

Jason