Re: Enabling peer to peer device transactions for PCIe devices

[Serguei Sagalovitch]

On 2016-11-24 11:26 AM, Jason Gunthorpe wrote:
> On Thu, Nov 24, 2016 at 10:45:18AM +0100, Christian König wrote:
> > Am 24.11.2016 um 00:25 schrieb Jason Gunthorpe:
> > > There is certainly nothing about the hardware that cares
> > > about ZONE_DEVICE vs System memory.
> > Well that is clearly not so simple. When your ZONE_DEVICE pages describe a
> > PCI BAR and another PCI device initiates a DMA to this address the DMA
> > subsystem must be able to check if the interconnection really works.
> I said the hardware doesn't care.. You are right, we still have an
> outstanding problem in Linux of how to generically DMA map a P2P
> address - which is a different issue from getting the P2P address from
> a __user pointer...
>
> Jason
I agreed but the problem is that one issue immediately introduce another 
one
to solve and so on (if we do not want to cut corners). I would think  that
a lot of them interconnected because the way how one problem could be
solved may impact solution for another.

btw: about "DMA map a p2p address": Right now to enable  p2p between 
devices
it is required/recommended to disable iommu support  (e.g. intel iommu 
driver
has special logic for graphics and  comment "Reserve all PCI MMIO to avoid
peer-to-peer access").