Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

[Christian König]

Am 22.06.21 um 17:11 schrieb Jason Gunthorpe:
> On Tue, Jun 22, 2021 at 04:12:26PM +0300, Oded Gabbay wrote:
>
> > > 1) Setting sg_page to NULL
> > > 2) 'mapping' pages for P2P DMA without going through the iommu
> > > 3) Allowing P2P DMA without using the p2p dma API to validate that it
> > >     can work at all in the first place.
> > >
> > > All of these result in functional bugs in certain system
> > > configurations.
> > >
> > > Jason
> > Hi Jason,
> > Thanks for the feedback.
> > Regarding point 1, why is that a problem if we disable the option to
> > mmap the dma-buf from user-space ?
> Userspace has nothing to do with needing struct pages or not
>
> Point 1 and 2 mostly go together, you supporting the iommu is not nice
> if you dont have struct pages.
>
> You should study Logan's patches I pointed you at as they are solving
> exactly this problem.
>
> > In addition, I didn't see any problem with sg_page being NULL in the
> > RDMA p2p dma-buf code. Did I miss something here ?
> No, the design of the dmabuf requires the exporter to do the dma maps
> and so it is only the exporter that is wrong to omit all the iommu and
> p2p logic.
>
> RDMA is OK today only because nobody has implemented dma buf support
> in rxe/si - mainly because the only implementations of exporters don't
> set the struct page and are thus buggy.
>
> > I will take two GAUDI devices and use one as an exporter and one as an
> > importer. I want to see that the solution works end-to-end, with real
> > device DMA from importer to exporter.
> I can tell you it doesn't. Stuffing physical addresses directly into
> the sg list doesn't involve any of the IOMMU code so any configuration
> that requires IOMMU page table setup will not work.

Sure it does. See amdgpu_vram_mgr_alloc_sgt:

        amdgpu_res_first(res, offset, length, &cursor);
        for_each_sgtable_sg((*sgt), sg, i) {
                phys_addr_t phys = cursor.start + adev->gmc.aper_base;
                size_t size = cursor.size;
                dma_addr_t addr;

                addr = dma_map_resource(dev, phys, size, dir,
                                        DMA_ATTR_SKIP_CPU_SYNC);
                r = dma_mapping_error(dev, addr);
                if (r)
                        goto error_unmap;

                sg_set_page(sg, NULL, size, 0);
                sg_dma_address(sg) = addr;
                sg_dma_len(sg) = size;

                amdgpu_res_next(&cursor, cursor.size);
        }

dma_map_resource() does the IOMMU mapping for us.

Regards,
Christian.


> Jason