Re: [RFC 0/8] Copy Offload with Peer-to-Peer PCI Memory

[Benjamin Herrenschmidt]

On Fri, 2017-04-14 at 21:37 +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2017-04-13 at 22:40 -0600, Logan Gunthorpe wrote:
> > 
> > On 13/04/17 10:16 PM, Jason Gunthorpe wrote:
> > > I'd suggest just detecting if there is any translation in bus
> > > addresses anywhere and just hard disabling P2P on such systems.
> > 
> > That's a fantastic suggestion. It simplifies things significantly.
> > Unless there are any significant objections I think I will plan on
> > doing
> > that.
> 
> I object.

Note: It would also make your stuff fundamentally incompatible with
KVM guest pass-through since KVM plays with remapping BARs all over
the place.

Ben.

> > > On modern hardware with 64 bit BARs there is very little reason
> > > to
> > > have translation, so I think this is a legacy feature.
> > 
> > Yes, p2pmem users are likely to be designing systems around it (ie
> > JBOFs) and not trying to shoehorn it onto legacy architectures.
> > 
> > At the very least, it makes sense to leave it out and if someone
> > comes
> > along who cares they can put in the effort to support the address
> > translation.
> > 
> > Thanks,
> > 
> > Logan