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SubjectRe: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory
On Tue, 13 Mar 2018 10:43:55 -0600
Logan Gunthorpe <logang@deltatee.com> wrote:

> On 12/03/18 09:28 PM, Sinan Kaya wrote:
> > On 3/12/2018 3:35 PM, Logan Gunthorpe wrote:
> > Regarding the switch business, It is amazing how much trouble you went into
> > limit this functionality into very specific hardware.
> >
> > I thought that we reached to an agreement that code would not impose
> > any limits on what user wants.
> >
> > What happened to all the emails we exchanged?
>
> It turns out that root ports that support P2P are far less common than
> anyone thought. So it will likely have to be a white list.

This came as a bit of a surprise to our PCIe architect.

His follow up was whether it was worth raising an ECR for the PCIe spec
to add a capability bit to allow this to be discovered. This might
long term avoid the need to maintain the white list for new devices.

So is it worth having a long term solution for making this discoverable?

Jonathan

> Nobody else
> seems keen on allowing the user to enable this on hardware that doesn't
> work. The easiest solution is still limiting it to using a switch. From
> there, if someone wants to start creating a white-list then that's
> probably the way forward to support root ports.
>
> And there's also the ACS problem which means if you want to use P2P on
> the root ports you'll have to disable ACS on the entire system. (Or
> preferably, the IOMMU groups need to get more sophisticated to allow for
> dynamic changes).
>
> Additionally, once you allow for root ports you may find the IOMMU
> getting in the way.
>
> So there are great deal more issues to sort out if you don't restrict to
> devices behind switches.
>
> Logan

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