Thanks, Benjamin, for the summary of some of the issues. On 14/04/17 04:07 PM, Benjamin Herrenschmidt wrote > So I assume the p2p code provides a way to address that too via special > dma_ops ? Or wrappers ? Not at this time. We will probably need a way to ensure the iommus do not attempt to remap these addresses. Though if it does, I'd expect everything would still work you just wouldn't get the performance or traffic flow you are looking for. We've been testing with the software iommu which doesn't have this problem. > The problem is that the latter while seemingly easier, is also slower > and not supported by all platforms and architectures (for example, > POWER currently won't allow it, or rather only allows a store-only > subset of it under special circumstances). Yes, I think situations where we have to cross host bridges will remain unsupported by this work for a long time. There are two many cases where it just doesn't work or it performs too poorly to be useful. > I don't fully understand how p2pmem "solves" that by creating struct > pages. The offset problem is one issue. But there's the iommu issue as > well, the driver cannot just use the normal dma_map ops. We are not using a proper iommu and we are dealing with systems that have zero offset. This case is also easily supported. I expect fixing the iommus to not map these addresses would also be reasonably achievable. Logan
