On Wed, Jun 26, 2019 at 02:45:38PM -0600, Logan Gunthorpe wrote: > > > On 2019-06-26 2:21 p.m., Jason Gunthorpe wrote: > > On Wed, Jun 26, 2019 at 12:31:08PM -0600, Logan Gunthorpe wrote: > >>> we have a hole behind len where we could store flag. Preferably > >>> optionally based on a P2P or other magic memory types config > >>> option so that 32-bit systems with 32-bit phys_addr_t actually > >>> benefit from the smaller and better packing structure. > >> > >> That seems sensible. The one thing that's unclear though is how to get > >> the PCI Bus address when appropriate. Can we pass that in instead of the > >> phys_addr with an appropriate flag? Or will we need to pass the actual > >> physical address and then, at the map step, the driver has to some how > >> lookup the PCI device to figure out the bus offset? > > > > I agree with CH, if we go down this path it is a layering violation > > for the thing injecting bio's into the block stack to know what struct > > device they egress&dma map on just to be able to do the dma_map up > > front. > > Not sure I agree with this statement. The p2pdma code already *must* > know and access the pci_dev of the dma device ahead of when it submits > the IO to know if it's valid to allocate and use P2P memory at all. I don't think we should make drives do that. What if it got CMB memory on some other device? > > For instance we could use a small hash table of the upper phys addr > > bits, or an interval tree, to do the lookup. > > Yes, if we're going to take a hard stance on this. But using an interval > tree (or similar) is a lot more work for the CPU to figure out these > mappings that may not be strictly necessary if we could just pass better > information down from the submitting driver to the mapping driver. Right, this is coming down to an optimization argument. I think there are very few cases (Basically yours) where the caller will know this info, so we need to support the other cases anyhow. I think with some simple caching this will become negligible for cases you care about Jason
