On Tue, Apr 18, 2017 at 2:03 PM, Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx> wrote: > On Tue, Apr 18, 2017 at 12:48:35PM -0700, Dan Williams wrote: > >> > Yes, I noticed this problem too and that makes sense. It just means >> > every dma_ops will probably need to be modified to either support p2p >> > pages or fail on them. Though, the only real difficulty there is that it >> > will be a lot of work. >> >> I don't think you need to go touch all dma_ops, I think you can just >> arrange for devices that are going to do dma to get redirected to a >> p2p aware provider of operations that overrides the system default >> dma_ops. I.e. just touch get_dma_ops(). > > I don't follow, when does get_dma_ops() return a p2p aware provider? > It has no way to know if the DMA is going to involve p2p, get_dma_ops > is called with the device initiating the DMA. > > So you'd always return the P2P shim on a system that has registered > P2P memory? > > Even so, how does this shim work? dma_ops are not really intended to > be stacked. How would we make unmap work, for instance? What happens > when the underlying iommu dma ops actually natively understands p2p > and doesn't want the shim? > > I think this opens an even bigger can of worms.. No, I don't think it does. You'd only shim when the target page is backed by a device, not host memory, and you can figure this out by a is_zone_device_page()-style lookup.
