On Mon, Jun 24, 2019 at 10:07:56AM -0600, Logan Gunthorpe wrote: > > For one passing a dma_addr_t through the block layer is a layering > > violation, and one that I think will also bite us in practice. > > The host physical to PCIe bus address mapping can have offsets, and > > those offsets absolutely can be different for differnet root ports. > > So with your caller generated dma_addr_t everything works fine with > > a switched setup as the one you are probably testing on, but on a > > sufficiently complicated setup with multiple root ports it can break. > > I don't follow this argument. Yes, I understand PCI Bus offsets and yes > I understand that they only apply beyond the bus they're working with. > But this isn't *that* complicated and it should be the responsibility of > the P2PDMA code to sort out and provide a dma_addr_t for. The dma_addr_t > that's passed through the block layer could be a bus address or it could > be the result of a dma_map_* request (if the transaction is found to go > through an RC) depending on the requirements of the devices being used. You assume all addressing is done by the PCI bus address. If a device is addressing its own BAR there is no reason to use the PCI bus address, as it might have much more intelligent schemes (usually bar + offset). > > > Also duplicating the whole block I/O stack, including hooks all over > > the fast path is pretty much a no-go. > > There was very little duplicate code in the patch set. (Really just the > mapping code). There are a few hooks, but in practice not that many if > we ignore the WARN_ONs. We might be able to work to reduce this further. > The main hooks are: when we skip bouncing, when we skip integrity prep, > when we split, and when we map. And the patchset drops the PCI_P2PDMA > hook when we map. So we're talking about maybe three or four extra ifs > that would likely normally be fast due to the branch predictor. And all of those add code to the block layer fast path.
