On 2025-06-11 10:41, Keith Busch wrote: > On Wed, Jun 11, 2025 at 10:39:17AM -0600, Logan Gunthorpe wrote: >> On 2025-06-11 10:26, Keith Busch wrote: >> That is all correct. In order to use P2P on a switch, with the IOMMU >> enabled, it is currently required to disable ACS for the devices in >> question. This is done with the command line parameter disable_acs_redir >> or config_acs. > > Is there some other mechansim that ensures a host memory mapped IOVA > doesn't collide with a PCI bus address then? Yes, in the absence of a switch with ACS protection this can be a problem. I haven't looked at this in a long time, but the iommu drivers reserve regions where the PCI addresses are valid so no iova will be allocated with a similar bus address. After a quick search, I believe today, this is handled by iova_reserve_pci_windows(). Logan
