On Tue, 13 Mar 2018 10:43:55 -0600 Logan Gunthorpe <logang@xxxxxxxxxxxx> 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
