[RFC PATCH v2 0/3] RDMA: add dma-buf support

[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 at amd.com>
> > > 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://elixir.bootlin.com/linux/latest/source/drivers/dma-buf/dma-buf.c#L811
> 
> 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?

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.

Cheers, Daniel

> Regards,
> Christian.
> 
> > 
> > Jianxin
> > 
> > > Christian.
> > > 
> > > > Jason
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch