Multi-process QEMU — QEMU documentation

This is the design document for multi-process QEMU. It does not necessarily reflect the status of the current implementation, which may lack features or be considerably different from what is described in this document. This document is still useful as a description of the goals and general direction of this feature.

Please refer to the following wiki for latest details: https://wiki.qemu.org/Features/MultiProcessQEMU

# Multi-process QEMU

QEMU is often used as the hypervisor for virtual machines running in the Oracle cloud. Since one of the advantages of cloud computing is the ability to run many VMs from different tenants in the same cloud infrastructure, a guest that compromised its hypervisor could potentially use the hypervisor’s access privileges to access data it is not authorized for.

QEMU can be susceptible to security attacks because it is a large, monolithic program that provides many features to the VMs it services. Many of these features can be configured out of QEMU, but even a reduced configuration QEMU has a large amount of code a guest can potentially attack. Separating QEMU reduces the attack surface by aiding to limit each component in the system to only access the resources that it needs to perform its job.

## QEMU services

QEMU can be broadly described as providing three main services. One is a VM control point, where VMs can be created, migrated, re-configured, and destroyed. A second is to emulate the CPU instructions within the VM, often accelerated by HW virtualization features such as Intel’s VT extensions. Finally, it provides IO services to the VM by emulating HW IO devices, such as disk and network devices.

### A multi-process QEMU

A multi-process QEMU involves separating QEMU services into separate host processes. Each of these processes can be given only the privileges it needs to provide its service, e.g., a disk service could be given access only to the disk images it provides, and not be allowed to access other files, or any network devices. An attacker who compromised this service would not be able to use this exploit to access files or devices beyond what the disk service was given access to.

A QEMU control process would remain, but in multi-process mode, will have no direct interfaces to the VM. During VM execution, it would still provide the user interface to hot-plug devices or live migrate the VM.

A first step in creating a multi-process QEMU is to separate IO services from the main QEMU program, which would continue to provide CPU emulation. i.e., the control process would also be the CPU emulation process. In a later phase, CPU emulation could be separated from the control process.

## Separating IO services

Separating IO services into individual host processes is a good place to begin for a couple of reasons. One is the sheer number of IO devices QEMU can emulate provides a large surface of interfaces which could potentially be exploited, and, indeed, have been a source of exploits in the past. Another is the modular nature of QEMU device emulation code provides interface points where the QEMU functions that perform device emulation can be separated from the QEMU functions that manage the emulation of guest CPU instructions. The devices emulated in the separate process are referred to as remote devices.

### QEMU device emulation

QEMU uses an object oriented SW architecture for device emulation code. Configured objects are all compiled into the QEMU binary, then objects are instantiated by name when used by the guest VM. For example, the code to emulate a device named “foo” is always present in QEMU, but its instantiation code is only run when the device is included in the target VM. (e.g., via the QEMU command line as -device foo)

The object model is hierarchical, so device emulation code names its parent object (such as “pci-device” for a PCI device) and QEMU will instantiate a parent object before calling the device’s instantiation code.

### Current separation models

In order to separate the device emulation code from the CPU emulation code, the device object code must run in a different process. There are a couple of existing QEMU features that can run emulation code separately from the main QEMU process. These are examined below.

#### vhost user model

Virtio guest device drivers can be connected to vhost user applications in order to perform their IO operations. This model uses special virtio device drivers in the guest and vhost user device objects in QEMU, but once the QEMU vhost user code has configured the vhost user application, mission-mode IO is performed by the application. The vhost user application is a daemon process that can be contacted via a known UNIX domain socket.

##### vhost socket

As mentioned above, one of the tasks of the vhost device object within QEMU is to contact the vhost application and send it configuration information about this device instance. As part of the configuration process, the application can also be sent other file descriptors over the socket, which then can be used by the vhost user application in various ways, some of which are described below.

##### vhost MMIO store acceleration

VMs are often run using HW virtualization features via the KVM kernel driver. This driver allows QEMU to accelerate the emulation of guest CPU instructions by running the guest in a virtual HW mode. When the guest executes instructions that cannot be executed by virtual HW mode, execution returns to the KVM driver so it can inform QEMU to emulate the instructions in SW.

One of the events that can cause a return to QEMU is when a guest device driver accesses an IO location. QEMU then dispatches the memory operation to the corresponding QEMU device object. In the case of a vhost user device, the memory operation would need to be sent over a socket to the vhost application. This path is accelerated by the QEMU virtio code by setting up an eventfd file descriptor that the vhost application can directly receive MMIO store notifications from the KVM driver, instead of needing them to be sent to the QEMU process first.

##### vhost interrupt acceleration

Another optimization used by the vhost application is the ability to directly inject interrupts into the VM via the KVM driver, again, bypassing the need to send the interrupt back to the QEMU process first. The QEMU virtio setup code configures the KVM driver with an eventfd that triggers the device interrupt in the guest when the eventfd is written. This irqfd file descriptor is then passed to the vhost user application program.

##### vhost access to guest memory

The vhost application is also allowed to directly access guest memory, instead of needing to send the data as messages to QEMU. This is also done with file descriptors sent to the vhost user application by QEMU. These descriptors can be passed to `mmap()` by the vhost application to map the guest address space into the vhost application.

IOMMUs introduce another level of complexity, since the address given to the guest virtio device to DMA to or from is not a guest physical address. This case is handled by having vhost code within QEMU register as a listener for IOMMU mapping changes. The vhost application maintains a cache of IOMMMU translations: sending translation requests back to QEMU on cache misses, and in turn receiving flush requests from QEMU when mappings are purged.

##### applicability to device separation

Much of the vhost model can be re-used by separated device emulation. In particular, the ideas of using a socket between QEMU and the device emulation application, using a file descriptor to inject interrupts into the VM via KVM, and allowing the application to `mmap()` the guest should be re used.

There are, however, some notable differences between how a vhost application works and the needs of separated device emulation. The most basic is that vhost uses custom virtio device drivers which always trigger IO with MMIO stores. A separated device emulation model must work with existing IO device models and guest device drivers. MMIO loads break vhost store acceleration since they are synchronous - guest progress cannot continue until the load has been emulated. By contrast, stores are asynchronous, the guest can continue after the store event has been sent to the vhost application.

Another difference is that in the vhost user model, a single daemon can support multiple QEMU instances. This is contrary to the security regime desired, in which the emulation application should only be allowed to access the files or devices the VM it’s running on behalf of can access. #### qemu-io model

Qemu-io is a test harness used to test changes to the QEMU block backend object code. (e.g., the code that implements disk images for disk driver emulation) Qemu-io is not a device emulation application per se, but it does compile the QEMU block objects into a separate binary from the main QEMU one. This could be useful for disk device emulation, since its emulation applications will need to include the QEMU block objects.

## New separation model based on proxy objects

A different model based on proxy objects in the QEMU program communicating with remote emulation programs could provide separation while minimizing the changes needed to the device emulation code. The rest of this section is a discussion of how a proxy object model would work.

### Remote emulation processes

The remote emulation process will run the QEMU object hierarchy without modification. The device emulation objects will be also be based on the QEMU code, because for anything but the simplest device, it would not be a tractable to re-implement both the object model and the many device backends that QEMU has.

The processes will communicate with the QEMU process over UNIX domain sockets. The processes can be executed either as standalone processes, or be executed by QEMU. In both cases, the host backends the emulation processes will provide are specified on its command line, as they would be for QEMU. For example:

    disk-proc -blockdev driver=file,node-name=file0,filename=disk-file0  \
    -blockdev driver=qcow2,node-name=drive0,file=file0

would indicate process disk-proc uses a qcow2 emulated disk named file0 as its backend.

Emulation processes may emulate more than one guest controller. A common configuration might be to put all controllers of the same device class (e.g., disk, network, etc.) in a single process, so that all backends of the same type can be managed by a single QMP monitor.

#### communication with QEMU

The first argument to the remote emulation process will be a Unix domain socket that connects with the Proxy object. This is a required argument.

    disk-proc <socket number> <backend list>

#### remote process QMP monitor

Remote emulation processes can be monitored via QMP, similar to QEMU itself. The QMP monitor socket is specified the same as for a QEMU process:

    disk-proc -qmp unix:/tmp/disk-mon,server

can be monitored over the UNIX socket path /tmp/disk-mon.

### QEMU command line

Each remote device emulated in a remote process on the host is represented as a -device of type pci-proxy-dev. A socket sub-option to this option specifies the Unix socket that connects to the remote process. An id sub-option is required, and it should be the same id as used in the remote process.

    qemu-system-x86_64 ... -device pci-proxy-dev,id=lsi0,socket=3

can be used to add a device emulated in a remote process

### QEMU management of remote processes

QEMU is not aware of the type of type of the remote PCI device. It is a pass through device as far as QEMU is concerned.

#### communication with emulation process

##### primary channel

The primary channel (referred to as com in the code) is used to bootstrap the remote process. It is also used to pass on device-agnostic commands like reset.

##### per-device channels

Each remote device communicates with QEMU using a dedicated communication channel. The proxy object sets up this channel using the primary channel during its initialization.

### QEMU device proxy objects

QEMU has an object model based on sub-classes inherited from the “object” super-class. The sub-classes that are of interest here are the “device” and “bus” sub-classes whose child sub-classes make up the device tree of a QEMU emulated system.

The proxy object model will use device proxy objects to replace the device emulation code within the QEMU process. These objects will live in the same place in the object and bus hierarchies as the objects they replace. i.e., the proxy object for an LSI SCSI controller will be a sub-class of the “pci-device” class, and will have the same PCI bus parent and the same SCSI bus child objects as the LSI controller object it replaces.

It is worth noting that the same proxy object is used to mediate with all types of remote PCI devices.

#### object initialization

The Proxy device objects are initialized in the exact same manner in which any other QEMU device would be initialized.

In addition, the Proxy objects perform the following two tasks: - Parses the “socket” sub option and connects to the remote process using this channel - Uses the “id” sub-option to connect to the emulated device on the separate process

##### class_init

The `class_init()` method of a proxy object will, in general behave similarly to the object it replaces, including setting any static properties and methods needed by the proxy.

##### instance_init / realize

The `instance_init()` and `realize()` functions would only need to perform tasks related to being a proxy, such are registering its own MMIO handlers, or creating a child bus that other proxy devices can be attached to later.

Other tasks will be device-specific. For example, PCI device objects will initialize the PCI config space in order to make a valid PCI device tree within the QEMU process.

#### address space registration

Most devices are driven by guest device driver accesses to IO addresses or ports. The QEMU device emulation code uses QEMU’s memory region function calls (such as `memory_region_init_io()`) to add callback functions that QEMU will invoke when the guest accesses the device’s areas of the IO address space. When a guest driver does access the device, the VM will exit HW virtualization mode and return to QEMU, which will then lookup and execute the corresponding callback function.

A proxy object would need to mirror the memory region calls the actual device emulator would perform in its initialization code, but with its own callbacks. When invoked by QEMU as a result of a guest IO operation, they will forward the operation to the device emulation process.

#### PCI config space

PCI devices also have a configuration space that can be accessed by the guest driver. Guest accesses to this space is not handled by the device emulation object, but by its PCI parent object. Much of this space is read-only, but certain registers (especially BAR and MSI-related ones) need to be propagated to the emulation process.

##### PCI parent proxy

One way to propagate guest PCI config accesses is to create a “pci-device-proxy” class that can serve as the parent of a PCI device proxy object. This class’s parent would be “pci-device” and it would override the PCI parent’s `config_read()` and `config_write()` methods with ones that forward these operations to the emulation program.

#### interrupt receipt

A proxy for a device that generates interrupts will need to create a socket to receive interrupt indications from the emulation process. An incoming interrupt indication would then be sent up to its bus parent to be injected into the guest. For example, a PCI device object may use `pci_set_irq()`.

#### live migration

The proxy will register to save and restore any vmstate it needs over a live migration event. The device proxy does not need to manage the remote device’s vmstate; that will be handled by the remote process proxy (see below).

### QEMU remote device operation

Generic device operations, such as DMA, will be performed by the remote process proxy by sending messages to the remote process.

#### DMA operations

DMA operations would be handled much like vhost applications do. One of the initial messages sent to the emulation process is a guest memory table. Each entry in this table consists of a file descriptor and size that the emulation process can `mmap()` to directly access guest memory, similar to `vhost_user_set_mem_table()`. Note guest memory must be backed by file descriptors, such as when QEMU is given the -mem-path command line option.

#### IOMMU operations

When the emulated system includes an IOMMU, the remote process proxy in QEMU will need to create a socket for IOMMU requests from the emulation process. It will handle those requests with an `address_space_get_iotlb_entry()` call. In order to handle IOMMU unmaps, the remote process proxy will also register as a listener on the device’s DMA address space. When an IOMMU memory region is created within the DMA address space, an IOMMU notifier for unmaps will be added to the memory region that will forward unmaps to the emulation process over the IOMMU socket.

#### device hot-plug via QMP

An QMP “device_add” command can add a device emulated by a remote process. It will also have “rid” option to the command, just as the -device command line option does. The remote process may either be one started at QEMU startup, or be one added by the “add-process” QMP command described above. In either case, the remote process proxy will forward the new device’s JSON description to the corresponding emulation process.

#### live migration

The remote process proxy will also register for live migration notifications with `vmstate_register()`. When called to save state, the proxy will send the remote process a secondary socket file descriptor to save the remote process’s device vmstate over. The incoming byte stream length and data will be saved as the proxy’s vmstate. When the proxy is resumed on its new host, this vmstate will be extracted, and a secondary socket file descriptor will be sent to the new remote process through which it receives the vmstate in order to restore the devices there.

### device emulation in remote process

The parts of QEMU that the emulation program will need include the object model; the memory emulation objects; the device emulation objects of the targeted device, and any dependent devices; and, the device’s backends. It will also need code to setup the machine environment, handle requests from the QEMU process, and route machine-level requests (such as interrupts or IOMMU mappings) back to the QEMU process.

#### initialization

The process initialization sequence will follow the same sequence followed by QEMU. It will first initialize the backend objects, then device emulation objects. The JSON descriptions sent by the QEMU process will drive which objects need to be created.

- address spaces

Before the device objects are created, the initial address spaces and memory regions must be configured with `memory_map_init()`. This creates a RAM memory region object (system_memory) and an IO memory region object (system_io).

- RAM

RAM memory region creation will follow how `pc_memory_init()` creates them, but must use `memory_region_init_ram_from_fd()` instead of `memory_region_allocate_system_memory()`. The file descriptors needed will be supplied by the guest memory table from above. Those RAM regions would then be added to the system_memory memory region with `memory_region_add_subregion()`.

- PCI

IO initialization will be driven by the JSON descriptions sent from the QEMU process. For a PCI device, a PCI bus will need to be created with `pci_root_bus_new()`, and a PCI memory region will need to be created and added to the system_memory memory region with `memory_region_add_subregion_overlap()`. The overlap version is required for architectures where PCI memory overlaps with RAM memory.

#### MMIO handling

The device emulation objects will use `memory_region_init_io()` to install their MMIO handlers, and `pci_register_bar()` to associate those handlers with a PCI BAR, as they do within QEMU currently.

In order to use `address_space_rw()` in the emulation process to handle MMIO requests from QEMU, the PCI physical addresses must be the same in the QEMU process and the device emulation process. In order to accomplish that, guest BAR programming must also be forwarded from QEMU to the emulation process.

#### interrupt injection

When device emulation wants to inject an interrupt into the VM, the request climbs the device’s bus object hierarchy until the point where a bus object knows how to signal the interrupt to the guest. The details depend on the type of interrupt being raised.

- PCI pin interrupts

On x86 systems, there is an emulated IOAPIC object attached to the root PCI bus object, and the root PCI object forwards interrupt requests to it. The IOAPIC object, in turn, calls the KVM driver to inject the corresponding interrupt into the VM. The simplest way to handle this in an emulation process would be to setup the root PCI bus driver (via `pci_bus_irqs()`) to send a interrupt request back to the QEMU process, and have the device proxy object reflect it up the PCI tree there.

- PCI MSI/X interrupts

PCI MSI/X interrupts are implemented in HW as DMA writes to a CPU-specific PCI address. In QEMU on x86, a KVM APIC object receives these DMA writes, then calls into the KVM driver to inject the interrupt into the VM. A simple emulation process implementation would be to send the MSI DMA address from QEMU as a message at initialization, then install an address space handler at that address which forwards the MSI message back to QEMU.

#### DMA operations

When a emulation object wants to DMA into or out of guest memory, it first must use dma_memory_map() to convert the DMA address to a local virtual address. The emulation process memory region objects setup above will be used to translate the DMA address to a local virtual address the device emulation code can access.

#### IOMMU

When an IOMMU is in use in QEMU, DMA translation uses IOMMU memory regions to translate the DMA address to a guest physical address before that physical address can be translated to a local virtual address. The emulation process will need similar functionality.

- IOTLB cache

The emulation process will maintain a cache of recent IOMMU translations (the IOTLB). When the translate() callback of an IOMMU memory region is invoked, the IOTLB cache will be searched for an entry that will map the DMA address to a guest PA. On a cache miss, a message will be sent back to QEMU requesting the corresponding translation entry, which be both be used to return a guest address and be added to the cache.

- IOTLB purge

The IOMMU emulation will also need to act on unmap requests from QEMU. These happen when the guest IOMMU driver purges an entry from the guest’s translation table.

#### live migration

When a remote process receives a live migration indication from QEMU, it will set up a channel using the received file descriptor with `qio_channel_socket_new_fd()`. This channel will be used to create a QEMUfile that can be passed to `qemu_save_device_state()` to send the process’s device state back to QEMU. This method will be reversed on restore - the channel will be passed to `qemu_loadvm_state()` to restore the device state.

### Accelerating device emulation

The messages that are required to be sent between QEMU and the emulation process can add considerable latency to IO operations. The optimizations described below attempt to ameliorate this effect by allowing the emulation process to communicate directly with the kernel KVM driver. The KVM file descriptors created would be passed to the emulation process via initialization messages, much like the guest memory table is done. #### MMIO acceleration

Vhost user applications can receive guest virtio driver stores directly from KVM. The issue with the eventfd mechanism used by vhost user is that it does not pass any data with the event indication, so it cannot handle guest loads or guest stores that carry store data. This concept could, however, be expanded to cover more cases.

The expanded idea would require a new type of KVM device: KVM_DEV_TYPE_USER. This device has two file descriptors: a master descriptor that QEMU can use for configuration, and a slave descriptor that the emulation process can use to receive MMIO notifications. QEMU would create both descriptors using the KVM driver, and pass the slave descriptor to the emulation process via an initialization message.

#### data structures

- guest physical range

The guest physical range structure describes the address range that a device will respond to. It includes the base and length of the range, as well as which bus the range resides on (e.g., on an x86machine, it can specify whether the range refers to memory or IO addresses).

A device can have multiple physical address ranges it responds to (e.g., a PCI device can have multiple BARs), so the structure will also include an enumerated identifier to specify which of the device’s ranges is being referred to.

Name | Description
--- | ---
addr | range base address
len | range length
bus | addr type (memory or IO)
id | range ID (e.g., PCI BAR)

- MMIO request structure

This structure describes an MMIO operation. It includes which guest physical range the MMIO was within, the offset within that range, the MMIO type (e.g., load or store), and its length and data. It also includes a sequence number that can be used to reply to the MMIO, and the CPU that issued the MMIO.

Name | Description
--- | ---
rid | range MMIO is within
offset | offset withing rid
type | e.g., load or store
len | MMIO length
data | store data
seq | sequence ID

- MMIO request queues

MMIO request queues are FIFO arrays of MMIO request structures. There are two queues: pending queue is for MMIOs that haven’t been read by the emulation program, and the sent queue is for MMIOs that haven’t been acknowledged. The main use of the second queue is to validate MMIO replies from the emulation program.

- scoreboard

Each CPU in the VM is emulated in QEMU by a separate thread, so multiple MMIOs may be waiting to be consumed by an emulation program and multiple threads may be waiting for MMIO replies. The scoreboard would contain a wait queue and sequence number for the per-CPU threads, allowing them to be individually woken when the MMIO reply is received from the emulation program. It also tracks the number of posted MMIO stores to the device that haven’t been replied to, in order to satisfy the PCI constraint that a load to a device will not complete until all previous stores to that device have been completed.

- device shadow memory

Some MMIO loads do not have device side-effects. These MMIOs can be completed without sending a MMIO request to the emulation program if the emulation program shares a shadow image of the device’s memory image with the KVM driver.

The emulation program will ask the KVM driver to allocate memory for the shadow image, and will then use `mmap()` to directly access it. The emulation program can control KVM access to the shadow image by sending KVM an access map telling it which areas of the image have no side-effects (and can be completed immediately), and which require a MMIO request to the emulation program. The access map can also inform the KVM drive which size accesses are allowed to the image.

#### master descriptor

The master descriptor is used by QEMU to configure the new KVM device. The descriptor would be returned by the KVM driver when QEMU issues a KVM_CREATE_DEVICE `ioctl()` with a KVM_DEV_TYPE_USER type.

KVM_DEV_TYPE_USER device ops

The KVM_DEV_TYPE_USER operations vector will be registered by a `kvm_register_device_ops()` call when the KVM system in initialized by `kvm_init()`. These device ops are called by the KVM driver when QEMU executes certain `ioctl()` operations on its KVM file descriptor. They include:

- create

This routine is called when QEMU issues a KVM_CREATE_DEVICE `ioctl()` on its per-VM file descriptor. It will allocate and initialize a KVM user device specific data structure, and assign the kvm_device private field to it.

- ioctl

This routine is invoked when QEMU issues an `ioctl()` on the master descriptor. The `ioctl()` commands supported are defined by the KVM device type. KVM_DEV_TYPE_USER ones will need several commands:

KVM_DEV_USER_SLAVE_FD creates the slave file descriptor that will be passed to the device emulation program. Only one slave can be created by each master descriptor. The file operations performed by this descriptor are described below.

The KVM_DEV_USER_PA_RANGE command configures a guest physical address range that the slave descriptor will receive MMIO notifications for. The range is specified by a guest physical range structure argument. For buses that assign addresses to devices dynamically, this command can be executed while the guest is running, such as the case when a guest changes a device’s PCI BAR registers.

KVM_DEV_USER_PA_RANGE will use `kvm_io_bus_register_dev()` to register kvm_io_device_ops callbacks to be invoked when the guest performs a MMIO operation within the range. When a range is changed, `kvm_io_bus_unregister_dev()` is used to remove the previous instantiation.

KVM_DEV_USER_TIMEOUT will configure a timeout value that specifies how long KVM will wait for the emulation process to respond to a MMIO indication.

- destroy

This routine is called when the VM instance is destroyed. It will need to destroy the slave descriptor; and free any memory allocated by the driver, as well as the kvm_device structure itself.

#### slave descriptor

The slave descriptor will have its own file operations vector, which responds to system calls on the descriptor performed by the device emulation program.

- read

A read returns any pending MMIO requests from the KVM driver as MMIO request structures. Multiple structures can be returned if there are multiple MMIO operations pending. The MMIO requests are moved from the pending queue to the sent queue, and if there are threads waiting for space in the pending to add new MMIO operations, they will be woken here.

- write

A write also consists of a set of MMIO requests. They are compared to the MMIO requests in the sent queue. Matches are removed from the sent queue, and any threads waiting for the reply are woken. If a store is removed, then the number of posted stores in the per-CPU scoreboard is decremented. When the number is zero, and a non side-effect load was waiting for posted stores to complete, the load is continued.

- ioctl

There are several ioctl()s that can be performed on the slave descriptor.

A KVM_DEV_USER_SHADOW_SIZE `ioctl()` causes the KVM driver to allocate memory for the shadow image. This memory can later be `mmap()`ed by the emulation process to share the emulation’s view of device memory with the KVM driver.

A KVM_DEV_USER_SHADOW_CTRL `ioctl()` controls access to the shadow image. It will send the KVM driver a shadow control map, which specifies which areas of the image can complete guest loads without sending the load request to the emulation program. It will also specify the size of load operations that are allowed.

- poll

An emulation program will use the `poll()` call with a POLLIN flag to determine if there are MMIO requests waiting to be read. It will return if the pending MMIO request queue is not empty.

- mmap

This call allows the emulation program to directly access the shadow image allocated by the KVM driver. As device emulation updates device memory, changes with no side-effects will be reflected in the shadow, and the KVM driver can satisfy guest loads from the shadow image without needing to wait for the emulation program.

#### kvm_io_device ops

Each KVM per-CPU thread can handle MMIO operation on behalf of the guest VM. KVM will use the MMIO’s guest physical address to search for a matching kvm_io_device to see if the MMIO can be handled by the KVM driver instead of exiting back to QEMU. If a match is found, the corresponding callback will be invoked.

- read

This callback is invoked when the guest performs a load to the device. Loads with side-effects must be handled synchronously, with the KVM driver putting the QEMU thread to sleep waiting for the emulation process reply before re-starting the guest. Loads that do not have side-effects may be optimized by satisfying them from the shadow image, if there are no outstanding stores to the device by this CPU. PCI memory ordering demands that a load cannot complete before all older stores to the same device have been completed.

- write

Stores can be handled asynchronously unless the pending MMIO request queue is full. In this case, the QEMU thread must sleep waiting for space in the queue. Stores will increment the number of posted stores in the per-CPU scoreboard, in order to implement the PCI ordering constraint above.

#### interrupt acceleration

This performance optimization would work much like a vhost user application does, where the QEMU process sets up eventfds that cause the device’s corresponding interrupt to be triggered by the KVM driver. These irq file descriptors are sent to the emulation process at initialization, and are used when the emulation code raises a device interrupt.

##### intx acceleration

Traditional PCI pin interrupts are level based, so, in addition to an irq file descriptor, a re-sampling file descriptor needs to be sent to the emulation program. This second file descriptor allows multiple devices sharing an irq to be notified when the interrupt has been acknowledged by the guest, so they can re-trigger the interrupt if their device has not de-asserted its interrupt.

intx irq descriptor

The irq descriptors are created by the proxy object `using event_notifier_init()` to create the irq and re-sampling eventds, and `kvm_vm_ioctl(KVM_IRQFD)` to bind them to an interrupt. The interrupt route can be found with `pci_device_route_intx_to_irq()`.

intx routing changes

Intx routing can be changed when the guest programs the APIC the device pin is connected to. The proxy object in QEMU will use `pci_device_set_intx_routing_notifier()` to be informed of any guest changes to the route. This handler will broadly follow the VFIO interrupt logic to change the route: de-assigning the existing irq descriptor from its route, then assigning it the new route. (see `vfio_intx_update()`)

##### MSI/X acceleration

MSI/X interrupts are sent as DMA transactions to the host. The interrupt data contains a vector that is programmed by the guest, A device may have multiple MSI interrupts associated with it, so multiple irq descriptors may need to be sent to the emulation program.

MSI/X irq descriptor

This case will also follow the VFIO example. For each MSI/X interrupt, an eventfd is created, a virtual interrupt is allocated by `kvm_irqchip_add_msi_route()`, and the virtual interrupt is bound to the eventfd with `kvm_irqchip_add_irqfd_notifier()`.

MSI/X config space changes

The guest may dynamically update several MSI-related tables in the device’s PCI config space. These include per-MSI interrupt enables and vector data. Additionally, MSIX tables exist in device memory space, not config space. Much like the BAR case above, the proxy object must look at guest config space programming to keep the MSI interrupt state consistent between QEMU and the emulation program.

## Disaggregated CPU emulation

After IO services have been disaggregated, a second phase would be to separate a process to handle CPU instruction emulation from the main QEMU control function. There are no object separation points for this code, so the first task would be to create one.

## Host access controls

Separating QEMU relies on the host OS’s access restriction mechanisms to enforce that the differing processes can only access the objects they are entitled to. There are a couple types of mechanisms usually provided by general purpose OSs.

### Discretionary access control

Discretionary access control allows each user to control who can access their files. In Linux, this type of control is usually too coarse for QEMU separation, since it only provides three separate access controls: one for the same user ID, the second for users IDs with the same group ID, and the third for all other user IDs. Each device instance would need a separate user ID to provide access control, which is likely to be unwieldy for dynamically created VMs.

### Mandatory access control

Mandatory access control allows the OS to add an additional set of controls on top of discretionary access for the OS to control. It also adds other attributes to processes and files such as types, roles, and categories, and can establish rules for how processes and files can interact.

#### Type enforcement

Type enforcement assigns a type attribute to processes and files, and allows rules to be written on what operations a process with a given type can perform on a file with a given type. QEMU separation could take advantage of type enforcement by running the emulation processes with different types, both from the main QEMU process, and from the emulation processes of different classes of devices.

For example, guest disk images and disk emulation processes could have types separate from the main QEMU process and non-disk emulation processes, and the type rules could prevent processes other than disk emulation ones from accessing guest disk images. Similarly, network emulation processes can have a type separate from the main QEMU process and non-network emulation process, and only that type can access the host tun/tap device used to provide guest networking.

#### Category enforcement

Category enforcement assigns a set of numbers within a given range to the process or file. The process is granted access to the file if the process’s set is a superset of the file’s set. This enforcement can be used to separate multiple instances of devices in the same class.

For example, if there are multiple disk devices provides to a guest, each device emulation process could be provisioned with a separate category. The different device emulation processes would not be able to access each other’s backing disk images.

Alternatively, categories could be used in lieu of the type enforcement scheme described above. In this scenario, different categories would be used to prevent device emulation processes in different classes from accessing resources assigned to other classes.