On Tue, Jun 22, 2021 at 11:42:27AM +0300, Oded Gabbay wrote: > On Tue, Jun 22, 2021 at 9:37 AM Christian König > <ckoenig.leichtzumerken@xxxxxxxxx> wrote: > > > > Am 22.06.21 um 01:29 schrieb Jason Gunthorpe: > > > On Mon, Jun 21, 2021 at 10:24:16PM +0300, Oded Gabbay wrote: > > > > > >> Another thing I want to emphasize is that we are doing p2p only > > >> through the export/import of the FD. We do *not* allow the user to > > >> mmap the dma-buf as we do not support direct IO. So there is no access > > >> to these pages through the userspace. > > > Arguably mmaping the memory is a better choice, and is the direction > > > that Logan's series goes in. Here the use of DMABUF was specifically > > > designed to allow hitless revokation of the memory, which this isn't > > > even using. > > > > The major problem with this approach is that DMA-buf is also used for > > memory which isn't CPU accessible. That isn't an issue here because the memory is only intended to be used with P2P transfers so it must be CPU accessible. > > That was one of the reasons we didn't even considered using the mapping > > memory approach for GPUs. Well, now we have DEVICE_PRIVATE memory that can meet this need too.. Just nobody has wired it up to hmm_range_fault() > > > So you are taking the hit of very limited hardware support and reduced > > > performance just to squeeze into DMABUF.. > > Thanks Jason for the clarification, but I honestly prefer to use > DMA-BUF at the moment. > It gives us just what we need (even more than what we need as you > pointed out), it is *already* integrated and tested in the RDMA > subsystem, and I'm feeling comfortable using it as I'm somewhat > familiar with it from my AMD days. You still have the issue that this patch is doing all of this P2P stuff wrong - following the already NAK'd AMD approach. > I'll go and read Logan's patch-set to see if that will work for us in > the future. Please remember, as Daniel said, we don't have struct page > backing our device memory, so if that is a requirement to connect to > Logan's work, then I don't think we will want to do it at this point. It is trivial to get the struct page for a PCI BAR. Jason
