On 18/04/17 04:50 PM, Dan Williams wrote: > On Tue, Apr 18, 2017 at 3:48 PM, Logan Gunthorpe <logang@xxxxxxxxxxxx> wrote: >> >> >> On 18/04/17 04:28 PM, Dan Williams wrote: >>> Unlike the pci bus address offset case which I think is fundamental to >>> support since shipping archs do this today, I think it is ok to say >>> p2p is restricted to a single sgl that gets to talk to host memory or >>> a single device. That said, what's wrong with a p2p aware map_sg >>> implementation calling up to the host memory map_sg implementation on >>> a per sgl basis? >> >> I think Ben said they need mixed sgls and that is where this gets messy. >> I think I'd prefer this too given trying to enforce all sgs in a list to >> be one type or another could be quite difficult given the state of the >> scatterlist code. >> >>>> Also, what happens if p2p pages end up getting passed to a device that >>>> doesn't have the injected dma_ops? >>> >>> This goes back to limiting p2p to a single pci host bridge. If the p2p >>> capability is coordinated with the bridge rather than between the >>> individual devices then we have a central point to catch this case. >> >> Not really relevant. If these pages get to userspace (as people seem >> keen on doing) or a less than careful kernel driver they could easily >> get into the dma_map calls of devices that aren't even pci related (via >> an O_DIRECT operation on an incorrect file or something). The common >> code must reject these and can't rely on an injected dma op. > > No, we can't do that at get_user_pages() time, it will always need to > be up to the device driver to fail dma that it can't perform. I'm not sure I follow -- are you agreeing with me? The dma_map_* needs to fail for any dma it cannot perform. Which means either all dma_ops providers need to be p2p aware or this logic has to be in dma_map_* itself. My point being: you can't rely on an injected dma_op for some devices to handle the fail case globally. -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
