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

[Jason Gunthorpe]

On Wed, Jul 01, 2020 at 05:42:21PM +0200, Daniel Vetter wrote:
> > >> All you need is the ability to stop wait for ongoing accesses to end and
> > >> make sure that new ones grab a new mapping.
> > > Swap and flush isn't a general HW ability either..
> > >
> > > I'm unclear how this could be useful, it is guarenteed to corrupt
> > > in-progress writes?
> > >
> > > Did you mean pause, swap and resume? That's ODP.
> >
> > Yes, something like this. And good to know, never heard of ODP.
> 
> Hm I thought ODP was full hw page faults at an individual page
> level,

Yes

> and this stop&resume is for the entire nic. Under the hood both apply
> back-pressure on the network if a transmission can't be received,
> but

NIC's don't do stop and resume, blocking the Rx pipe is very
problematic and performance destroying.

The strategy for something like ODP is more complex, and so far no NIC
has deployed it at any granularity larger than per-page.

> So since Jason really doesn't like dma_fence much I think for rdma
> synchronous it is. And it shouldn't really matter, since waiting for a
> small transaction to complete at rdma wire speed isn't really that
> long an operation. 

Even if DMA fence were to somehow be involved, how would it look?

Jason