That's what an IOMMU does. Without an IOMMU devices on extension buses directly address physical memory. With an MMU the host CPU creates a virtual address sp

That's what an IOMMU does. Without an IOMMU devices on extension buses directly address physical memory. With an MMU the host CPU creates a virtual address space for devices and can thus limit access to the main memory (conveniently also allows passing devices to VM guests), much like virtual memory for processes/VMs.