Re: [PATCH v5 1/5] RDMA/umem: Support importing dma-buf as user memory region

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Oct 17, 2020 at 12:57:21AM +0000, Xiong, Jianxin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Friday, October 16, 2020 5:28 PM
> > To: Xiong, Jianxin <jianxin.xiong@intel.com>
> > Cc: linux-rdma@vger.kernel.org; dri-devel@lists.freedesktop.org; Doug Ledford <dledford@redhat.com>; Leon Romanovsky
> > <leon@kernel.org>; Sumit Semwal <sumit.semwal@linaro.org>; Christian Koenig <christian.koenig@amd.com>; Vetter, Daniel
> > <daniel.vetter@intel.com>
> > Subject: Re: [PATCH v5 1/5] RDMA/umem: Support importing dma-buf as user memory region
> > 
> > On Thu, Oct 15, 2020 at 03:02:45PM -0700, Jianxin Xiong wrote:
> > > +struct ib_umem *ib_umem_dmabuf_get(struct ib_device *device,
> > > +				   unsigned long addr, size_t size,
> > > +				   int dmabuf_fd, int access,
> > > +				   const struct ib_umem_dmabuf_ops *ops) {
> > > +	struct dma_buf *dmabuf;
> > > +	struct ib_umem_dmabuf *umem_dmabuf;
> > > +	struct ib_umem *umem;
> > > +	unsigned long end;
> > > +	long ret;
> > > +
> > > +	if (check_add_overflow(addr, (unsigned long)size, &end))
> > > +		return ERR_PTR(-EINVAL);
> > > +
> > > +	if (unlikely(PAGE_ALIGN(end) < PAGE_SIZE))
> > > +		return ERR_PTR(-EINVAL);
> > > +
> > > +	if (unlikely(!ops || !ops->invalidate || !ops->update))
> > > +		return ERR_PTR(-EINVAL);
> > > +
> > > +	umem_dmabuf = kzalloc(sizeof(*umem_dmabuf), GFP_KERNEL);
> > > +	if (!umem_dmabuf)
> > > +		return ERR_PTR(-ENOMEM);
> > > +
> > > +	umem_dmabuf->ops = ops;
> > > +	INIT_WORK(&umem_dmabuf->work, ib_umem_dmabuf_work);
> > > +
> > > +	umem = &umem_dmabuf->umem;
> > > +	umem->ibdev = device;
> > > +	umem->length = size;
> > > +	umem->address = addr;
> > 
> > addr here is offset within the dma buf, but this code does nothing with it.
> > 
> The current code assumes 0 offset, and 'addr' is the nominal starting address of the
> buffer. If this is to be changed to offset, then yes, some more handling is needed
> as you mentioned below.

There is no such thing as 'nominal starting address'

If the user is to provide any argument it can only be offset and length.

> > Also, dma_buf_map_attachment() does not do the correct dma mapping
> > for RDMA, eg it does not use ib_dma_map(). This is not a problem
> > for mlx5 but it is troublesome to put in the core code.
> 
> ib_dma_map() uses dma_map_single(), GPU drivers use dma_map_resource() for
> dma_buf_map_attachment(). They belong to the same family, but take different
> address type (kernel address vs MMIO physical address). Could you elaborate what
> the problem could be for non-mlx5 HCAs?

They use the virtual dma ops which we intend to remove

Jason