The problem is that, used properly, IOMMUs are horribly expensive.
Consider a NIC driver where you're mapping an outgoing packet for DMA. What used to essentially be a virtual to physical translation becomes a virt to phys + entering the phys in the iommu + removing the mapping when the transmit is complete. This is expensive for hardware and software reasons. At one point I benchmarked a 100g setup on linux, and with the IOMMU enabled, we lost about 90% of the bandwidth and most of the CPU time was spent in lock contention over the red-black tree that managed the IOMMU tables. This was 5-ish years ago, so perhaps things have gotten better.
So that makes people want to just enable the IOMMU for SR-IOV (and full device) pass-thru to VMs. This is cheaper, since you just set the mapping up when you allocate phys mem for the guest, and tear them down when freeing phys mem.
MacOS used to use a really cool trick where they pre-mapped all mbufs into the IOMMU. That made network traffic transmit and receive comparatively fast. However,it also prevented lots of optimizations that modern operating systems use for zero-copy IO (like attaching pages from sendfile directly to mbufs, similer to skb_frags).
Consider a NIC driver where you're mapping an outgoing packet for DMA. What used to essentially be a virtual to physical translation becomes a virt to phys + entering the phys in the iommu + removing the mapping when the transmit is complete. This is expensive for hardware and software reasons. At one point I benchmarked a 100g setup on linux, and with the IOMMU enabled, we lost about 90% of the bandwidth and most of the CPU time was spent in lock contention over the red-black tree that managed the IOMMU tables. This was 5-ish years ago, so perhaps things have gotten better.
So that makes people want to just enable the IOMMU for SR-IOV (and full device) pass-thru to VMs. This is cheaper, since you just set the mapping up when you allocate phys mem for the guest, and tear them down when freeing phys mem.
MacOS used to use a really cool trick where they pre-mapped all mbufs into the IOMMU. That made network traffic transmit and receive comparatively fast. However,it also prevented lots of optimizations that modern operating systems use for zero-copy IO (like attaching pages from sendfile directly to mbufs, similer to skb_frags).