Re: [PATCH v7 07/12] dma-mapping: introduce dma_has_iommu()

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

On Mon, Oct 09, 2017 at 01:18:20PM -0600, Jason Gunthorpe wrote:
> > > If RDMA is driving this need, why not invalidate backing RDMA MRs
> > > instead of requiring a IOMMU to do it? RDMA MR are finer grained and
> > > do not suffer from the re-use problem David W. brought up with IOVAs..
> > 
> > Sounds promising. All I want in the end is to be sure that the kernel
> > is enabled to stop any in-flight RDMA at will without asking
> > userspace. Does this require per-RDMA driver opt-in or is there a
> > common call that can be made?
> 
> I don't think this has ever come up in the context of an all-device MR
> invalidate requirement. Drivers already have code to invalidate
> specifc MRs, but to find all MRs that touch certain pages and then
> invalidate them would be new code.

The whole point is that we should not need that IFF we provide the
right interface.

If we have a new 'register memory with a lease', the driver (or in fact
probably the umem core for the drivers using it) has the lease associated
with the ib_umem structure, which will just need a backpointer from the
ib_umem to the to the MR to unregister it.

Which might be a good opportunity to break the user MR from the in-kernel
ones and merge it with ib_umem, but that's a different story..
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