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

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Feb 03, 2023 at 08:31:52AM -0400, Jason Gunthorpe wrote:
> On Fri, Feb 03, 2023 at 10:10:45AM +0000, Tian, Kevin wrote:
> > > From: Nicolin Chen <nicolinc@nvidia.com>
> > > 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

Will change that in v2.

Thanks
Nic