Hi Jason, On Thu, 7 Oct 2021 08:59:18 -0300, Jason Gunthorpe <j...@nvidia.com> wrote:
> On Fri, Oct 08, 2021 at 12:54:52AM +1300, Barry Song wrote: > > On Fri, Oct 8, 2021 at 12:32 AM Jason Gunthorpe <j...@nvidia.com> wrote: > > > > > > > > On Thu, Oct 07, 2021 at 06:43:33PM +1300, Barry Song wrote: > > > > > > > So do we have a case where devices can directly access the kernel's > > > > data structure such as a list/graph/tree with pointers to a kernel > > > > virtual address? then devices don't need to translate the address > > > > of pointers in a structure. I assume this is one of the most useful > > > > features userspace SVA can provide. > > > > > > AFIACT that is the only good case for KVA, but it is also completely > > > against the endianess, word size and DMA portability design of the > > > kernel. > > > > > > Going there requires some new set of portable APIs for gobally > > > coherent KVA dma. > > > > yep. I agree. it would be very weird if accelerators/gpu are sharing > > kernel' data struct, but for each "DMA" operation - reading or writing > > the data struct, we have to call dma_map_single/sg or > > dma_sync_single_for_cpu/device etc. It seems once devices and cpus > > are sharing virtual address(SVA), code doesn't need to do explicit > > map/sync each time. > That is what we have today with sva_bind_device. > No, it still need to do something to manage visibility from the > current CPU to the DMA - it might not be flushing a cache, but it is > probably a arch specific CPU barrier instruction. > Are you talking about iommu_dma_sync_single_for_cpu(), this is not SVA specific, right? On platforms that are DMA snooped, this barrier is not needed. But I think your point is that once we convert to DMA API, the sync/barrier is covered by DMA APIs if !dev_is_dma_coherent(dev). Then all archs are good. We could also add a check for dev_is_dma_coherent(dev) before using SVA. > Jason Thanks, Jacob _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu