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

[Tian, Kevin]

> 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?