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+# QEMU Command-Line Architecture
+
+> **Status:** Migration in progress — see the [phase tracker](#migration-phases) below.
+> **Issues:** [#12187](https://github.com/kata-containers/kata-containers/issues/12187) (refactor),
+>             [#12125](https://github.com/kata-containers/kata-containers/issues/12125) (NUMA / hugepages)
+
+> **Living document:** This file is updated with every PR that lands a migration
+> phase.  It intentionally contains open questions and decision notes while the
+> migration is in progress.  Once Phase 6 completes, all such notes will be
+> removed and the document will serve as the definitive, stable reference for the
+> QEMU command-line architecture — no TODOs, no decision stubs, just the design
+> as implemented.
+
+## Overview
+
+This document describes the target machine-centric architecture for the QEMU
+command-line generator in `runtime-rs`, why the current design needs changing,
+and how each migration phase moves the code toward the target.
+
+The [Grace Platform Configurations](#grace-platform-configurations) section
+enumerates 7 concrete command-line topologies derived from tested production
+deployments.  Each becomes a golden test fixture; the architecture must be
+able to generate every one of them exactly from typed `Platform` inputs.
+
+---
+
+## Current State (the Problem)
+
+All QEMU command-line construction lives in `cmdline_generator.rs` (~3 700 lines).
+The design is inspired by `govmm` and works well for simple cases, but has
+accumulated three structural problems:
+
+### 1. Architecture decisions scattered across devices
+
+Whether a virtio device needs `bus=pcie.0` depends on the machine layout
+(Q35 with `pxb-pcie`, `virt` with extra root ports, NUMA enabled, …), yet every
+device struct encodes that decision itself via `#[cfg(target_arch = ...)]`:
+
+```rust
+// Only valid for Q35 or VIRT machine types aka x86 or aarch64
+#[cfg(any(target_arch = "x86_64", target_arch = "aarch64"))]
+params.push("bus=pcie.0".to_owned());
+```
+
+This pattern is duplicated across `DeviceVhostUserFs`, `DeviceVirtioBlk`,
+`VhostVsock`, `DeviceVirtioNet`, `DeviceVirtioSerial`, `DeviceVirtconsole`,
+`DeviceRng`, `DeviceIntelIommu`, and `DeviceVirtioScsi`.
+
+### 2. Stringly-typed flat `Machine` struct
+
+```rust
+struct Machine {
+    r#type: String,          // "q35", "virt", "s390-ccw-virtio", …
+    accel: String,
+    options: String,         // raw accelerator string from config
+    nvdimm: bool,
+    kernel_irqchip: Option<String>,
+    confidential_guest_support: String,
+    memory_backend: Option<String>,
+}
+```
+
+All machine types share one struct.  `kernel_irqchip` is meaningless on `virt`
+or `s390x`; `gic-version` is meaningful only on `virt`.  There is no type-level
+enforcement.
+
+### 3. No hugepages / NUMA-aware memory backends
+
+`runtime-rs` does not support `memory-backend-file` with `hugepages` today
+(tracked in [#12125](https://github.com/kata-containers/kata-containers/issues/12125)).
+`MemoryBackendFile` exists in `cmdline_generator.rs` but is only wired for
+`/dev/shm` (virtiofs shared memory) and nvdimm paths — never for the
+`/hugepages` mount needed to back the entire guest with huge pages.
+
+---
+
+## Target Architecture
+
+The refactor replaces the flat struct with a **machine-centric** model.  All
+topology decisions (PCIe bus assignment, IOMMU wiring, NUMA layout, memory
+backends) are made when constructing the `Machine` and `Platform`.  Devices
+receive a pre-resolved bus string and typed references to shared objects;
+they no longer branch on architecture.
+
+### Module layout (target)
+
+```
+src/runtime-rs/crates/hypervisor/src/qemu/
+├── ARCHITECTURE.md        ← this file
+├── cmdline_generator.rs   ← legacy; shrinks as phases complete
+├── inner.rs
+├── mod.rs
+├── qmp.rs
+└── machine/               ← new; introduced in Phase 0
+    ├── mod.rs             (Machine enum + Platform + PciTopology)
+    ├── q35.rs
+    ├── virt.rs
+    ├── pseries.rs
+    ├── s390x.rs
+    └── probe.rs           (PlatformProbe trait + HostTopology)
+```
+
+### Core types
+
+#### `Machine` — per-machine-type structs
+
+```rust
+pub enum Machine {
+    Q35(machine::Q35),
+    Virt(machine::Virt),
+    Pseries(machine::Pseries),
+    S390xCcwVirtio(machine::S390xCcwVirtio),
+}
+
+// Q35: intel-iommu is a global singleton on the machine, not per-bus.
+pub struct Q35 {
+    pub base: BaseMachine,
+    pub kernel_irqchip: Option<String>,
+    /// Global IOMMU device for Q35.  Emitted as a top-level -device intel-iommu,
+    /// not attached to any pxb-pcie.  Contrast with SmmuV3 on PciRootComplex.
+    pub intel_iommu: Option<IntelIommuConfig>,
+    pub runtime: RuntimeFeatures,
+}
+
+pub struct IntelIommuConfig {
+    pub intremap: bool,
+    pub caching_mode: bool,
+}
+
+// Virt: arm-smmuv3 is per-bus; it lives on PciRootComplex, not here.
+pub struct Virt {
+    pub base: BaseMachine,
+    pub gic_version: Option<u8>,
+    pub ras: bool,
+    /// Required for Grace GPU passthrough; must be a power of 2.
+    /// 4T for GH200/GB200 (≤4 GPUs), 8T for GB300 NVL72 (4 GPUs).
+    pub highmem_mmio_size: Option<u64>,
+    pub runtime: RuntimeFeatures,
+}
+```
+
+`kernel_irqchip` and `intel_iommu` live exclusively on `Q35`; `gic_version` and
+`highmem_mmio_size` live exclusively on `Virt`.  The compiler enforces this —
+no runtime guards needed.
+
+#### `PciTopology` — bus resolution moved here
+
+```rust
+pub struct PciTopology {
+    pub default_bus: Option<String>,    // "pcie.0" when NUMA / multi-RC is active
+    pub roots: Vec<PciRootComplex>,
+}
+
+pub struct PciRootComplex {
+    pub id: String,              // "pcie.N"
+    pub bus_nr: u8,
+    /// Maps to pxb-pcie `numa_node=N`.  Required on Grace; omitting it causes
+    /// "Unknown NUMA node; performance will be reduced" in the guest kernel.
+    pub numa_node: Option<u32>,
+    /// Bus-attached IOMMU (arm-smmuv3 on aarch64).  Intel IOMMU is a global
+    /// device on Q35 and lives on Machine::Q35, not here.
+    pub iommu: Option<BusIommu>,
+    /// One entry per passthrough device on this SMMU.
+    /// 1 root port = 1 GPU (1:1 SMMU mapping).
+    /// N root ports = N GPUs sharing the same physical SMMU.
+    pub root_ports: Vec<PciRootPort>,
+}
+
+pub struct PciRootPort {
+    pub id: String,       // "pcie.portN"
+    pub chassis: u8,
+    pub device: Option<VfioDevice>,
+}
+
+/// IOMMU that attaches to a specific PCIe expander bus (pxb-pcie).
+/// Intel IOMMU is a Q35-global device and is NOT represented here —
+/// see Machine::Q35::intel_iommu.
+pub enum BusIommu {
+    SmmuV3 {
+        accel: bool,
+        ats: bool,
+        pasid: bool,
+        oas: u8,
+        ril: bool,
+        /// Enable SMMU command-queue virtualisation (vCMDQ).  Requires
+        /// physically contiguous guest memory (hugepages or EGM).
+        cmdqv: bool,
+    },
+}
+```
+
+**SMMU grouping rule:** GPUs that share a physical SMMU on the host **must** be
+placed on the same `PciRootComplex` in the guest (they share the same
+`arm-smmuv3` device).  The IOMMU group boundaries in host sysfs determine the
+grouping.  See [Config 3](#config-3--4-gpus-2-gpus-per-smmu-33-numa-nodes) for
+the 2-GPUs-per-SMMU topology.
+
+#### `Objects` — shared QEMU `-object` backends
+
+```rust
+pub struct Objects {
+    pub iommufd: Option<IommufdBackend>,
+    pub memory_backends: Vec<MemoryBackend>,
+    pub thread_contexts: Vec<ThreadContext>,
+    pub acpi_links: Vec<AcpiPciNodeLink>,
+    pub rng: Option<ObjectRngRandom>,
+}
+
+pub enum MemoryBackend {
+    Ram  { id: String, size: u64 },
+    /// File-backed memory.  Two uses:
+    ///   path = "/dev/hugepages/" → hugepages-backed guest RAM (vCMDQ)
+    ///   path = "/dev/egmN"      → per-socket EGM region (vEGM)
+    File { id: String, size: u64, path: String, prealloc: bool, share: bool },
+}
+
+pub enum AcpiPciNodeLink {
+    /// Emitted 8× per passthrough GPU.  The GPU driver uses these nodes to
+    /// online GPU memory to the guest kernel (required for MIG regardless of
+    /// whether MIG is actually enabled).
+    GenericInitiator { id: String, pci_dev: String, node: u32 },
+    /// Emitted 1× per passthrough GPU.  Links the GPU to the per-socket EGM
+    /// memory-backend file.  `node` is the CpuMem NUMA node for the socket
+    /// that holds this GPU's EGM device, not a GPU initiator node.
+    EgmMemory        { id: String, pci_dev: String, node: u32 },
+}
+```
+
+**EGM is per socket, not per GPU:** one `MemoryBackend::File` with `/dev/egmN`
+per CPU socket.  Two GPUs on the same socket share the full EGM backing; each
+gets its own `acpi-egm-memory` pointing to that socket's CpuMem NUMA node.
+See [Config 7](#config-7--vegm-2-gpus-per-socket-4-gpus-2-sockets).
+
+#### `HostTopology` — probe result driving `Platform`
+
+```rust
+/// Read from host sysfs and IOMMU group layout before constructing Platform.
+pub struct HostTopology {
+    pub sockets: Vec<SocketInfo>,
+    pub gpu_smmu_groups: Vec<GpuSmmuGroup>,
+    pub egm_sockets: Vec<EgmSocketInfo>,
+}
+
+pub struct SocketInfo {
+    pub id: u32,
+    pub cpu_range: std::ops::Range<u32>,
+}
+
+/// All GPUs in this group share a physical SMMU and must be placed on the same
+/// pxb-pcie + arm-smmuv3 in the guest.  Derived from /sys/kernel/iommu_groups.
+pub struct GpuSmmuGroup {
+    pub pci_bus_addrs: Vec<String>,   // e.g. ["0008:06:00.0", "0009:06:00.0"]
+    pub socket: u32,
+}
+
+/// One entry per /dev/egmN device (created by the nvgrace-egm kernel module).
+pub struct EgmSocketInfo {
+    pub path: String,    // "/dev/egm4"
+    pub socket: u32,
+    pub total_size: u64,
+}
+```
+
+`Platform::apply_host_defaults(topo)` consumes `HostTopology` to populate
+`PciTopology::roots` (one `PciRootComplex` per `GpuSmmuGroup`) and
+`Objects::memory_backends` (one `MemoryBackend::File` per `EgmSocketInfo`).
+This is the **only** location that knows about DGX, GB300, or any host flavour.
+
+#### `Platform` — single wiring point
+
+```rust
+pub struct Platform {
+    pub machine: Machine,
+    pub pci: PciTopology,
+    pub objects: Objects,
+}
+
+impl Platform {
+    pub fn from_config(config: &HypervisorConfig) -> Result<Platform> { … }
+    pub fn apply_host_defaults(&mut self, topo: &HostTopology) { … }
+    pub fn with_hugepages(mut self, path: &str) -> Self { … }
+}
+```
+
+### NUMA Layout Rules
+
+The guest Linux kernel processes ACPI SRAT entries in a fixed order:
+
+1. **CPU Affinity** — nodes with a `cpus=` range (CpuMem nodes, one per socket)
+2. **Generic Affinity** — initiator nodes for PCIe devices (8 per GPU for MIG)
+3. **Memory-only Affinity** — nodes without CPUs (EGM backing, hotplug regions)
+
+The `-numa node` arguments **must appear in this order** in the QEMU command
+line.  Placing Generic Affinity nodes before CpuMem nodes causes the kernel to
+assign wrong NUMA node IDs.
+
+**8 NUMA nodes per GPU (MIG):** Each passthrough GPU requires exactly 8 dedicated
+generic-initiator NUMA nodes regardless of whether MIG is in use.  The GPU
+driver (CUDA) uses these nodes to online GPU memory to the guest kernel.  Total
+node count with 4 GPUs on a single-socket host: 1 CpuMem + 4 × 8 = 33 nodes.
+
+**Memory hotplug placeholder:** QEMU assigns the last-defined NUMA node to the
+hotplug region.  When DIMM/NVDIMM hotplug is enabled, always append one empty
+NUMA node after all real nodes so hotplug memory does not overlap GPU nodes.
+
+**GPU memory spill prevention:** GPU NUMA nodes may attract page migration from
+`autonuma` or systemd NUMA policies.  Mitigate with explicit NUMA distances:
+
+```text
+-numa dist,src=<gpu_node>,dst=<cpumem_node>,val=254
+```
+
+Or by disabling NUMA balancing in the guest OS.
+
+**`highmem-mmio-size` sizing on `-machine virt`:**
+- GH200 / GB200 with ≤ 4 GPUs → `4T`
+- GB300 NVL72 with 4 GPUs → `8T`
+- Must be a power of 2; round up to the next power when in doubt.
+
+### Hugepages wiring (`with_hugepages`)
+
+```rust
+pub fn with_hugepages(mut self, path: &str) -> Self {
+    let id = "m0".to_owned();
+    self.objects.memory_backends.push(MemoryBackend::File {
+        id: id.clone(),
+        size: self.machine.memory_size(),
+        path: path.to_owned(),   // "/dev/hugepages/"
+        prealloc: true,
+        share: true,
+    });
+    self.machine.set_memory_backend(&id);  // -machine memory-backend=m0
+    self
+}
+```
+
+This replaces the current pattern where `set_memory_backend_file` +
+`set_memory_backend` are called ad-hoc from `QemuCmdLine` internals.  The
+caller checks `config.memory_info.enable_hugepages` and calls
+`Platform::with_hugepages("/dev/hugepages/")` once.  No device code changes.
+
+EGM memory is natively physically contiguous, so vEGM implicitly satisfies the
+vCMDQ contiguity requirement without hugepages.  `cmdqv=on` is still required
+in the `arm-smmuv3` device args when vCMDQ is enabled.
+
+### Emission order
+
+`QemuCmdLine::build()` becomes a thin orchestrator:
+
+1. Emit `-object iommufd,id=iommufd0` first (all vfio devices and SMMU SID
+   tables reference it).
+2. Emit remaining `Objects` — `memory_backends`, `thread_contexts`, `rng` —
+   all `-object` lines, IDs defined before any reference.
+3. Emit `Machine` — picks up `memory-backend=` from objects, `highmem-mmio-size`.
+4. Emit **CpuMem** `-numa node` entries: one per socket, with `cpus=` + `memdev=`.
+5. Emit **GPU initiator** `-numa node` entries: 8 per GPU, no `cpus`/`memdev`,
+   ordered by GPU index.
+6. Emit **EGM / hotplug** `-numa node` entries: memory-only nodes, no `cpus`.
+7. Emit `PciTopology` in bus-number order: `pxb-pcie` (with `numa_node=`),
+   `arm-smmuv3`, root ports, vfio devices per root complex.
+8. Emit `Objects.acpi_links`: `acpi-generic-initiator` (8 per GPU) then
+   `acpi-egm-memory` (1 per GPU).  These reference device IDs emitted in step 7.
+
+Steps 4–6 must be in that order to match Linux ACPI SRAT processing.
+
+---
+
+## Grace Platform Configurations
+
+The 7 configurations below are derived from tested production deployments of
+NVIDIA Grace GPU passthrough.  Each becomes a golden test fixture in **Phase 0b**.
+The implementation must reproduce every one exactly from the corresponding
+`Platform` + `HostTopology` input.
+
+All Grace configurations share these constants:
+- `-device vfio-pci-nohotplug` (not `vfio-pci`) — required for C2C interconnect
+- `-object iommufd,id=iommufd0` — modern IOMMU fd interface; legacy VFIO groups
+  not supported on Grace
+- `arm-smmuv3` fixed parameters: `accel=on,ats=on,ril=off,pasid=on,oas=48`
+- Host kernel driver: `nvgrace-gpu-vfio-pci` (replaces standard `vfio-pci`)
+- EGM kernel module: `nvgrace-egm` (creates `/dev/egm*` character devices)
+
+### Config 1 — Single GPU, 1 SMMU (9 NUMA nodes)
+
+```text
+-object iommufd,id=iommufd0
+-object memory-backend-ram,size=16G,id=m0
+-machine virt,accel=kvm,gic-version=3,ras=on,highmem-mmio-size=4T,memory-backend=m0
+-numa node,memdev=m0,cpus=0-3,nodeid=0
+-numa node,nodeid=1
+...
+-numa node,nodeid=8
+-device pxb-pcie,id=pcie.1,bus_nr=1,bus=pcie.0,numa_node=0
+-device arm-smmuv3,primary-bus=pcie.1,id=smmuv3.1,accel=on,ats=on,ril=off,pasid=on,oas=48
+-device pcie-root-port,id=pcie.port1,bus=pcie.1,chassis=1,io-reserve=0
+-device vfio-pci-nohotplug,host=0008:06:00.0,bus=pcie.port1,rombar=0,id=dev0,iommufd=iommufd0
+-object acpi-generic-initiator,id=gi0,pci-dev=dev0,node=1
+...
+-object acpi-generic-initiator,id=gi7,pci-dev=dev0,node=8
+```
+
+`HostTopology`: 1 socket, 1 `GpuSmmuGroup { pci_bus_addrs: ["0008:06:00.0"], socket: 0 }`.
+
+### Config 2 — 4 GPUs, 1 GPU per SMMU (33 NUMA nodes)
+
+Each GPU gets its own `PciRootComplex` (one `pxb-pcie` + one `arm-smmuv3` + one
+root port).  Repeat the pxb-pcie/smmuv3/root-port/vfio block 4 times:
+
+```text
+-object iommufd,id=iommufd0
+-object memory-backend-ram,size=16G,id=m0
+-machine virt,...,highmem-mmio-size=4T,memory-backend=m0
+-numa node,memdev=m0,cpus=0-3,nodeid=0
+-numa node,nodeid=1 ... -numa node,nodeid=32   # 4×8 = 32 GPU initiator nodes
+
+# Per GPU (N = 1..4):
+-device pxb-pcie,id=pcie.N,bus_nr=N,bus=pcie.0,numa_node=0
+-device arm-smmuv3,primary-bus=pcie.N,id=smmuv3.N,accel=on,ats=on,ril=off,pasid=on,oas=48
+-device pcie-root-port,id=pcie.portN,bus=pcie.N,chassis=N,io-reserve=0
+-device vfio-pci-nohotplug,host=<addr>,bus=pcie.portN,rombar=0,id=dev<N-1>,iommufd=iommufd0
+-object acpi-generic-initiator,id=gi<8*(N-1)>,pci-dev=dev<N-1>,node=<1+8*(N-1)>
+...                                                               # ×8 per GPU
+```
+
+`HostTopology`: 1 socket, 4 `GpuSmmuGroup` each with 1 address.
+
+### Config 3 — 4 GPUs, 2 GPUs per SMMU (33 NUMA nodes)
+
+GPUs sharing a physical SMMU share one `PciRootComplex` with **2 root ports**.
+2 complexes × 2 GPUs each:
+
+```text
+-device pxb-pcie,id=pcie.1,bus_nr=1,bus=pcie.0,numa_node=0
+-device arm-smmuv3,primary-bus=pcie.1,id=smmuv3.1,accel=on,ats=on,ril=off,pasid=on,oas=48
+-device pcie-root-port,id=pcie.port1,bus=pcie.1,chassis=1,io-reserve=0
+-device vfio-pci-nohotplug,host=0008:06:00.0,bus=pcie.port1,rombar=0,id=dev0,iommufd=iommufd0
+-device pcie-root-port,id=pcie.port2,bus=pcie.1,chassis=2,io-reserve=0
+-device vfio-pci-nohotplug,host=0009:06:00.0,bus=pcie.port2,rombar=0,id=dev1,iommufd=iommufd0
+
+-device pxb-pcie,id=pcie.2,bus_nr=9,bus=pcie.0,numa_node=0
+-device arm-smmuv3,primary-bus=pcie.2,id=smmuv3.2,accel=on,ats=on,ril=off,pasid=on,oas=48
+-device pcie-root-port,id=pcie.port3,bus=pcie.2,chassis=3,io-reserve=0
+-device vfio-pci-nohotplug,host=0010:06:00.0,bus=pcie.port3,rombar=0,id=dev2,iommufd=iommufd0
+-device pcie-root-port,id=pcie.port4,bus=pcie.2,chassis=4,io-reserve=0
+-device vfio-pci-nohotplug,host=0011:06:00.0,bus=pcie.port4,rombar=0,id=dev3,iommufd=iommufd0
+
+-object acpi-generic-initiator,id=gi0,pci-dev=dev0,node=1
+...
+-object acpi-generic-initiator,id=gi7,pci-dev=dev0,node=8
+-object acpi-generic-initiator,id=gi8,pci-dev=dev1,node=9
+...
+-object acpi-generic-initiator,id=gi15,pci-dev=dev1,node=16
+# × 2 more for dev2, dev3
+```
+
+`HostTopology`: 1 socket, 2 `GpuSmmuGroup` each with 2 addresses.
+
+### Config 4 — GPU + NIC passthrough
+
+Same structure as Config 2 but one `PciRootPort` holds a NIC (`vfio-pci-nohotplug`
+with the NIC's PCI address).  That root port does **not** emit
+`acpi-generic-initiator` links — the NIC has no GPU memory to online.
+
+`VfioDevice` carries a `kind: VfioDeviceKind` field (enum `Gpu` / `Nic` / …)
+that gates initiator emission.  The NIC shares the host SMMU with no GPU on its
+bus, so it gets its own `PciRootComplex`.
+
+### Config 5 — vCMDQ (hugepages + SMMU command-queue virtualisation)
+
+Same PCIe topology as Config 1 or 2, but `MemoryBackend::Ram` is replaced with
+`MemoryBackend::File` for physically contiguous memory (required by the vCMDQ
+hardware for the queue base address), and `cmdqv=on` is added to `arm-smmuv3`:
+
+```text
+-object memory-backend-file,id=m0,size=16G,mem-path=/dev/hugepages/,prealloc=on,share=on
+-machine virt,...,memory-backend=m0
+-device arm-smmuv3,...,cmdqv=on
+```
+
+`Platform::with_hugepages("/dev/hugepages/")` + `IommuKind::SmmuV3 { cmdqv: true }`.
+
+### Config 6 — vEGM, 1 GPU per socket (4 GPUs, 4 sockets)
+
+One `memory-backend-file` per socket using the `/dev/egmN` device created by
+`nvgrace-egm`.  One `acpi-egm-memory` per GPU pointing to its socket's CpuMem
+NUMA node:
+
+```text
+-object memory-backend-file,id=m0,mem-path=/dev/egm4,size=56896M,share=on,prealloc=on
+-object memory-backend-file,id=m1,mem-path=/dev/egm5,size=56896M,share=on,prealloc=on
+-object memory-backend-file,id=m2,mem-path=/dev/egm6,size=56896M,share=on,prealloc=on
+-object memory-backend-file,id=m3,mem-path=/dev/egm7,size=56896M,share=on,prealloc=on
+-machine virt,...
+-numa node,memdev=m0,cpus=0,nodeid=0
+-numa node,memdev=m1,cpus=1,nodeid=1
+-numa node,memdev=m2,cpus=2,nodeid=2
+-numa node,memdev=m3,cpus=3,nodeid=3
+-numa node,nodeid=4 ... -numa node,nodeid=35   # 4×8 GPU initiator nodes
+
+# PCI topology: 4× (pxb-pcie + smmuv3 + root-port + vfio) — same shape as Config 2
+
+-object acpi-egm-memory,id=egm0,pci-dev=dev0,node=0   # GPU on socket 0
+-object acpi-egm-memory,id=egm1,pci-dev=dev1,node=1   # GPU on socket 1
+-object acpi-egm-memory,id=egm2,pci-dev=dev2,node=2
+-object acpi-egm-memory,id=egm3,pci-dev=dev3,node=3
+```
+
+`HostTopology`: 4 sockets, 4 `GpuSmmuGroup` (1 GPU each), 4 `EgmSocketInfo`.
+
+### Config 7 — vEGM, 2 GPUs per socket (4 GPUs, 2 sockets)
+
+Two GPUs per socket share the socket's EGM device.  The `/dev/egmN` path appears
+in one `memory-backend-file` at full socket size.  Both `acpi-egm-memory` entries
+for that socket point to the same CpuMem NUMA node:
+
+```text
+-object memory-backend-file,id=m0,mem-path=/dev/egm4,size=56896M,share=on,prealloc=on
+-object memory-backend-file,id=m1,mem-path=/dev/egm5,size=56896M,share=on,prealloc=on
+-machine virt,...
+-numa node,memdev=m0,cpus=0-1,nodeid=0
+-numa node,memdev=m1,cpus=2-3,nodeid=1
+-numa node,nodeid=2 ... -numa node,nodeid=33   # 4×8 GPU initiator nodes
+
+# PCI topology: 2× (pxb-pcie + smmuv3 + 2 root ports + 2 vfio) — Config 3 shape
+
+-object acpi-egm-memory,id=egm0,pci-dev=dev0,node=0   # both GPUs on socket 0 → node=0
+-object acpi-egm-memory,id=egm1,pci-dev=dev1,node=0
+-object acpi-egm-memory,id=egm2,pci-dev=dev2,node=1   # both GPUs on socket 1 → node=1
+-object acpi-egm-memory,id=egm3,pci-dev=dev3,node=1
+```
+
+`HostTopology`: 2 sockets, 2 `GpuSmmuGroup` (2 GPUs each), 2 `EgmSocketInfo`.
+
+---
+
+## Migration Phases
+
+Each phase is a self-contained PR.  Phases 0–1 introduce new types without
+touching the hot path; Phases 2–5 strangle the old code one device at a time.
+
+### Phase 0 — Test harness and empty data types
+
+- **0a** — golden-test harness + one trivial fixture (basic `virt` machine)
+- **0b** — All 7 Grace configurations as command-line fixtures + parse smoke test.
+  Each fixture provides the expected QEMU argument list and the `HostTopology`
+  input that produces it.  Zero implementation; tests all fail intentionally.
+- **0c** — empty `machine/` module with unit tests on pure helpers
+  (`format_memory`, `numa_node` string, bus-name helpers, etc.)
+
+No behaviour changes.  CI green throughout.
+
+### Phase 1 — Platform probe (unused)
+
+Introduce `PlatformProbe` trait, `HostTopology` struct, and `Platform::from_config`.
+Nothing in the hot path calls them yet.  Tests assert construction succeeds for
+each supported machine type and that `HostTopology` round-trips through
+`apply_host_defaults` without panic.
+
+### Phase 2 — Strangle bus resolution (one device per PR)
+
+For each device listed below, pass a resolved `bus: String` instead of
+computing it inside `ToQemuParams`:
+
+- `DeviceVhostUserFs`
+- `DeviceVirtioBlk`
+- `VhostVsock`
+- `DeviceVirtioNet`
+- `DeviceVirtioSerial`
+- `DeviceVirtconsole`
+- `DeviceRng`
+- `DeviceIntelIommu`
+- `DeviceVirtioScsi`
+
+Each PR removes one `#[cfg(target_arch)]` block and one duplicated comment.
+The golden fixtures validate that the emitted command lines are unchanged.
+
+Final PR in Phase 2: remove all remaining `#[cfg(target_arch)]` blocks from
+`cmdline_generator.rs`.
+
+### Phase 3 — Objects registry
+
+- Lift `MemoryBackendFile` into `Objects::memory_backends` as
+  `MemoryBackend::File`.
+- Wire hugepages via `Platform::with_hugepages`.
+- Lift `ObjectIoThread` / `ObjectRngRandom` into `Objects`.
+
+This is the phase that enables hugepages for `runtime-rs` (issue #12125).
+
+### Phase 4 — Multi-RC PCIe and NUMA layout
+
+- Emit `pxb-pcie` (with `numa_node=`) + per-RC `arm-smmuv3` from `PciTopology`.
+- Support N root ports per `PciRootComplex` (Config 3 shape: 2 GPUs per SMMU).
+- Add `VfioPciNoHotplug` with typed `IommufdRef` and `VfioDeviceKind`.
+- Emit NUMA nodes in the correct order (CpuMem → GPU initiators → memory-only).
+- `apply_host_defaults` wired end-to-end: Configs 1–4 golden fixtures pass.
+
+### Phase 5 — vCMDQ and vEGM
+
+- `SmmuV3 { cmdqv: true }` + `MemoryBackend::File { path: "/dev/hugepages/", … }`.
+- `AcpiPciNodeLink::EgmMemory` + per-socket `MemoryBackend::File { path: "/dev/egmN", … }`.
+- `EgmSocketInfo` probe wired into `apply_host_defaults`.
+- Configs 5–7 golden fixtures pass.
+
+### Phase 6 — Cleanup
+
+- Delete dead code from `cmdline_generator.rs`.
+- Remove feature flags / compat shims introduced during migration.
+- Final golden-test sweep across all 7 Grace configs plus existing machine types.
+- Remove all TODOs and decision stubs from this document.
+
+---
+
+## Design Principles
+
+1. **Machine decisions stay in `Machine` and `PciTopology`.**
+   Devices receive resolved values; they do not branch on architecture or
+   machine type.
+
+2. **Singletons are first-class.**
+   `iommufd0` is emitted once and referenced via `IommufdRef`.  No string
+   concatenation in device impls.
+
+3. **One location for host-specific wiring.**
+   `Platform::apply_host_defaults` is the only place that knows about DGX,
+   GB300, or any future host flavour.
+
+4. **IOMMU placement matches hardware placement.**
+   Bus-attached IOMMUs (`arm-smmuv3`) live on `PciRootComplex`.  Global IOMMUs
+   (`intel-iommu`) live on the machine type (`Machine::Q35`).  These are
+   different device placement models and must not share a field.
+   SMMU grouping: devices sharing a physical SMMU are placed on the same
+   `PciRootComplex`; devices on separate physical SMMUs get separate entries.
+
+5. **NUMA emission order is non-negotiable.**
+   CpuMem → GenericInitiator → MemoryOnly.  The `Platform` builder enforces
+   this by construction; there is no API to emit them in a different order.
+
+6. **Incremental migration, no flag day.**
+   Each phase leaves CI green.  Old `QemuCmdLine` and new `Platform` coexist
+   until the strangle is complete.
+
+7. **Types enforce constraints.**
+   `kernel_irqchip` compiles only on `Q35`.  `cmdqv` compiles only on
+   `IommuKind::SmmuV3`.  You cannot emit `gic-version` on a Q35 machine.
+
+---
+
+## Related Documents
+
+- [Issue #12187](https://github.com/kata-containers/kata-containers/issues/12187) — full design spec with data-model definitions and worked examples
+- [Issue #12125](https://github.com/kata-containers/kata-containers/issues/12125) — NUMA and hugepages roadmap