> -----Original Message-----
> From: Jason Gunthorpe <jgg@xxxxxxxx>
> Sent: Monday, October 05, 2020 6:13 AM
> To: Xiong, Jianxin <jianxin.xiong@xxxxxxxxx>
> Cc: linux-rdma@xxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx; Doug Ledford <dledford@xxxxxxxxxx>; Leon Romanovsky
> <leon@xxxxxxxxxx>; Sumit Semwal <sumit.semwal@xxxxxxxxxx>; Christian Koenig <christian.koenig@xxxxxxx>; Vetter, Daniel
> <daniel.vetter@xxxxxxxxx>
> Subject: Re: [RFC PATCH v3 1/4] RDMA/umem: Support importing dma-buf as user memory region
>
> On Sun, Oct 04, 2020 at 12:12:28PM -0700, Jianxin Xiong wrote:
> > Dma-buf is a standard cross-driver buffer sharing mechanism that can
> > be used to support peer-to-peer access from RDMA devices.
> >
> > Device memory exported via dma-buf is associated with a file descriptor.
> > This is passed to the user space as a property associated with the
> > buffer allocation. When the buffer is registered as a memory region,
> > the file descriptor is passed to the RDMA driver along with other
> > parameters.
> >
> > Implement the common code for importing dma-buf object and mapping
> > dma-buf pages.
> >
> > Signed-off-by: Jianxin Xiong <jianxin.xiong@xxxxxxxxx>
> > Reviewed-by: Sean Hefty <sean.hefty@xxxxxxxxx>
> > Acked-by: Michael J. Ruhl <michael.j.ruhl@xxxxxxxxx>
> > ---
> > drivers/infiniband/core/Makefile | 2 +-
> > drivers/infiniband/core/umem.c | 4 +
> > drivers/infiniband/core/umem_dmabuf.c | 291
> > ++++++++++++++++++++++++++++++++++
> > drivers/infiniband/core/umem_dmabuf.h | 14 ++
> > drivers/infiniband/core/umem_odp.c | 12 ++
> > include/rdma/ib_umem.h | 19 ++-
> > 6 files changed, 340 insertions(+), 2 deletions(-) create mode
> > 100644 drivers/infiniband/core/umem_dmabuf.c
> > create mode 100644 drivers/infiniband/core/umem_dmabuf.h
>
> I think this is using ODP too literally, dmabuf isn't going to need fine grained page faults, and I'm not sure this locking scheme is OK - ODP is
> horrifically complicated.
>
> If this is the approach then I think we should make dmabuf its own stand alone API, reg_user_mr_dmabuf()
That's the original approach in the first version. We can go back there.
>
> The implementation in mlx5 will be much more understandable, it would just do dma_buf_dynamic_attach() and program the XLT exactly
> the same as a normal umem.
>
> The move_notify() simply zap's the XLT and triggers a work to reload it after the move. Locking is provided by the dma_resv_lock. Only a
> small disruption to the page fault handler is needed.
>
We considered such scheme but didn't go that way due to the lack of notification when the move is done and thus the work wouldn't know when it can reload.
Now I think it again, we could probably signal the reload in the page fault handler.
> > + dma_resv_lock(umem_dmabuf->attach->dmabuf->resv, NULL);
> > + sgt = dma_buf_map_attachment(umem_dmabuf->attach,
> > + DMA_BIDIRECTIONAL);
> > + dma_resv_unlock(umem_dmabuf->attach->dmabuf->resv);
>
> This doesn't look right, this lock has to be held up until the HW is programmed
The mapping remains valid until being invalidated again. There is a sequence number checking before programming the HW.
>
> The use of atomic looks probably wrong as well.
Do you mean umem_dmabuf->notifier_seq? Could you elaborate the concern?
>
> > + k = 0;
> > + total_pages = ib_umem_odp_num_pages(umem_odp);
> > + for_each_sg(umem->sg_head.sgl, sg, umem->sg_head.nents, j) {
> > + addr = sg_dma_address(sg);
> > + pages = sg_dma_len(sg) >> page_shift;
> > + while (pages > 0 && k < total_pages) {
> > + umem_odp->dma_list[k++] = addr | access_mask;
> > + umem_odp->npages++;
> > + addr += page_size;
> > + pages--;
>
> This isn't fragmenting the sg into a page list properly, won't work for unaligned things
I thought the addresses are aligned, but will add explicit alignment here.
>
> And really we don't need the dma_list for this case, with a fixed whole mapping DMA SGL a normal umem sgl is OK and the normal umem
> XLT programming in mlx5 is fine.
The dma_list is used by both "polulate_mtt()" and "mlx5_ib_invalidate_range", which are used for XLT programming and invalidating (zapping), respectively.
>
> Jason
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