PCIe ACS Controls Whether Peer Traffic Can Bypass the IOMMU
PCIe ACS Controls Whether Peer Traffic Can Bypass the IOMMU An IOMMU can restrict DMA only for transactions that reach its translation and permission checks. PCI Express complicates that boundary because two endpoints under the same hierarchy can exchange peer-to-peer traffic without sending every transaction through the root complex. A switch may be able to route a request directly from one downstream port to another. PCIe Access Control Services (ACS) adds controls for that routing boundary. Depending on the component and supported ACS features, the fabric can validate a request, block selected translated traffic, redirect peer requests or completions upstream, or constrain their egress. The result is not merely a routing preference. ACS can determine whether two devices are separable at the IOMMU boundary.