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

[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.

Jianxin

> Christian.
> 
> >
> > Jason