Re: [RFC PATCH 1/3] RDMA/umem: Change for rdma devices has not dma device

From: Daniel Vetter
Date: Wed Sep 08 2021 - 15:22:56 EST

On Wed, Sep 8, 2021 at 3:33 PM Christian König <christian.koenig@xxxxxxx> wrote:
> Am 08.09.21 um 13:18 schrieb Jason Gunthorpe:
> > On Wed, Sep 08, 2021 at 05:41:39PM +0900, Shunsuke Mie wrote:
> >> 2021年9月8日(水) 16:20 Christoph Hellwig <hch@xxxxxxxxxxxxx>:
> >>> On Wed, Sep 08, 2021 at 04:01:14PM +0900, Shunsuke Mie wrote:
> >>>> Thank you for your comment.
> >>>>> On Wed, Sep 08, 2021 at 03:16:09PM +0900, Shunsuke Mie wrote:
> >>>>>> To share memory space using dma-buf, a API of the dma-buf requires dma
> >>>>>> device, but devices such as rxe do not have a dma device. For those case,
> >>>>>> change to specify a device of struct ib instead of the dma device.
> >>>>> So if dma-buf doesn't actually need a device to dma map why do we ever
> >>>>> pass the dma_device here? Something does not add up.
> >>>> As described in the dma-buf api guide [1], the dma_device is used by dma-buf
> >>>> exporter to know the device buffer constraints of importer.
> >>>> [1];;sdata=ARwQyo%2BCjMohaNbyREofToHIj2bndL5L0HaU9cOrYq4%3D&amp;reserved=0
> >>> Which means for rxe you'd also have to pass the one for the underlying
> >>> net device.
> >> I thought of that way too. In that case, the memory region is constrained by the
> >> net device, but rxe driver copies data using CPU. To avoid the constraints, I
> >> decided to use the ib device.
> > Well, that is the whole problem.
> >
> > We can't mix the dmabuf stuff people are doing that doesn't fill in
> > the CPU pages in the SGL with RXE - it is simply impossible as things
> > currently are for RXE to acess this non-struct page memory.
> Yeah, agree that doesn't make much sense.
> When you want to access the data with the CPU then why do you want to
> use DMA-buf in the first place?
> Please keep in mind that there is work ongoing to replace the sg table
> with an DMA address array and so make the underlying struct page
> inaccessible for importers.

Also if you do have a dma-buf, you can just dma_buf_vmap() the buffer
for cpu access. Which intentionally does not require any device. No
idea why there's a dma_buf_attach involved. Now not all exporters
support this, but that's fixable, and you must call
dma_buf_begin/end_cpu_access for cache management if the allocation
isn't cpu coherent. But it's all there, no need to apply hacks of
allowing a wrong device or other fun things.
Daniel Vetter
Software Engineer, Intel Corporation