On Wed, Jul 8, 2020 at 10:20 AM Christian König <christian.koenig@xxxxxxx> wrote: > > Am 08.07.20 um 11:49 schrieb Daniel Vetter: > > On Wed, Jul 08, 2020 at 11:38:31AM +0200, Christian König wrote: > >> Am 07.07.20 um 23:58 schrieb Xiong, Jianxin: > >>>> -----Original Message----- > >>>> From: Christian König <christian.koenig@xxxxxxx> > >>>> Am 03.07.20 um 15:14 schrieb Jason Gunthorpe: > >>>>> On Fri, Jul 03, 2020 at 02:52:03PM +0200, Daniel Vetter wrote: > >>>>> > >>>>>> So maybe I'm just totally confused about the rdma model. I thought: > >>>>>> - you bind a pile of memory for various transactions, that might > >>>>>> happen whenever. Kernel driver doesn't have much if any insight into > >>>>>> when memory isn't needed anymore. I think in the rdma world that's > >>>>>> called registering memory, but not sure. > >>>>> Sure, but once registered the memory is able to be used at any moment > >>>>> with no visibilty from the kernel. > >>>>> > >>>>> Unlike GPU the transactions that trigger memory access do not go > >>>>> through the kernel - so there is no ability to interrupt a command > >>>>> flow and fiddle with mappings. > >>>> This is the same for GPUs with user space queues as well. > >>>> > >>>> But we can still say for a process if that this process is using a DMA-buf which is moved out and so can't run any more unless the DMA-buf is > >>>> accessible again. > >>>> > >>>> In other words you somehow need to make sure that the hardware is not accessing a piece of memory any more when you want to move it. > >>>> > >>> While a process can be easily suspended, there is no way to tell the RDMA NIC not to process posted work requests that use specific memory regions (or with any other conditions). > >>> > >>> So far it appears to me that DMA-buf dynamic mapping for RDMA is only viable with ODP support. For NICs without ODP, a way to allow pinning the device memory is still needed. > >> And that's exactly the reason why I introduced explicit pin()/unpin() > >> functions into the DMA-buf API: > >> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.bootlin.com%2Flinux%2Flatest%2Fsource%2Fdrivers%2Fdma-buf%2Fdma-buf.c%23L811&data=02%7C01%7Cchristian.koenig%40amd.com%7C6d785861acc542a2f53608d823243a7c%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637297985792135311&sdata=bBrkDynlACE9DAIlGntxXhE1unr%2FBxw5IRTm6AtV6WQ%3D&reserved=0 > >> > >> It's just that at least our devices drivers currently prevent P2P with > >> pinned DMA-buf's for two main reasons: > >> > >> a) To prevent deny of service attacks because P2P BARs are a rather rare > >> resource. > >> > >> b) To prevent failures in configuration where P2P is not always possible > >> between all devices which want to access a buffer. > > So the above is more or less the question in the cover letter (which > > didn't make it to dri-devel). Can we somehow throw that limitation out, or > > is that simply not a good idea? > > At least for the AMD graphics drivers that's most certain not a good idea. > > We do have an use case where buffers need to be in system memory because > P2P doesn't work. > > And by pinning them to VRAM you can create a really nice deny of service > attack against the X system. > On the other hand, on modern platforms with large or resizable BARs, you may end up with systems with more vram than system ram. Alex > > Simply moving buffers to system memory when they're pinned does simplify a > > lot of headaches. For a specific custom built system we can avoid that > > maybe, but I think upstream is kinda a different thing. > > Yes, agree completely on that. Customers which are willing to take the > risk can easily do this themselves. > > But that is not something we should probably do for upstream. > > Regards, > Christian. > > > > > Cheers, Daniel > > > >> Regards, > >> Christian. > >> > >>> Jianxin > >>> > >>>> Christian. > >>>> > >>>>> Jason > > _______________________________________________ > dri-devel mailing list > dri-devel@xxxxxxxxxxxxxxxxxxxxx > https://lists.freedesktop.org/mailman/listinfo/dri-devel
