Multiple device emulators

Xen’s ioreq-server feature allows for several device emulator processes to be attached to the same domain, each emulating different sets of virtual hardware. This makes it possible, for example, to emulate network devices in a separate process for improved security and isolation, or to provide special purpose emulators for particular virtual hardware devices.

ioreq-server is currently used in XenServer to support vGPU, where it is configured via the legacy toolstack interface. These changes will make multiple emulators usable in open source Xen via the new libxl interface.

libxl changes

• The singleton device_model_version, device_model_stubdomain and device_model fields in the b_info structure will be replaced by a list of (version, stubdomain, model, arguments) tuples, one for each emulator.

• libxl_domain_create_new() will be changed to spawn a new device model for each entry in the list.

It may also be useful to spawn the device models separately and only attach them during domain creation. This could be supported by making each device_model entry a union of pid | parameter_tuple. If such an entry specifies a parameter tuple, it is processed as above; if it specifies a pid, libxl_domain_create_new(), the existing device model with that pid is attached instead.

QEMU changes

• Patches to make QEMU register with Xen as an ioreq-server have been submitted upstream, but not yet applied.

• QEMU’s --machine none and --nodefaults options should make it possible to create an empty machine and add just a host bus, PCI bus and device. This has not yet been fully demonstrated, so QEMU changes may be required.

Xen changes

• Until now, ioreq-server has only been used to connect one extra device model, in addition to the default one. Multiple emulators should work, but there is a chance that bugs will be discovered.

Interfacing with xenopsd

This functionality will only be available through the experimental Xenlight-based xenopsd.

• the VM_build clause in the atomics_of_operation function will be changed to fill in the list of emulators to be created (or attached) in the b_info struct

Host Configuration

vGPU support is implemented mostly in xenopsd, so no Xapi changes are required to support vGPU through the generic device model mechanism. Changes would be required if we decided to expose the additional device models through the API, but in the near future it is more likely that any additional device models will be dealt with entirely by xenopsd.