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

["Christian König" <christian.koenig@amd.com>]

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