1Dom0less
2========
3
4"Dom0less" is a set of Xen features that enable the deployment of a Xen
5system without an control domain (often referred to as "dom0"). Each
6feature can be used independently from the others, unless otherwise
7stated.
8
9Booting Multiple Domains from Device Tree
10-----------------------------------------
11
12This feature enables Xen to create a set of DomUs at boot time.
13Information about the DomUs to be created by Xen is passed to the
14hypervisor via Device Tree. Specifically, the existing Device Tree based
15Multiboot specification has been extended to allow for multiple domains
16to be passed to Xen. See docs/misc/arm/device-tree/booting.txt for more
17information about the Multiboot specification and how to use it.
18
19Currently, a control domain ("dom0") is still required to manage the DomU
20domains, but the system can start also without dom0 if the Device Tree
21doesn't specify the dom0 kernel and it declares one or more domUs.
22Instead of waiting for the control domain (when declared) to be fully
23booted and the Xen tools to become available, domains created by Xen
24this way are started right away in parallel. Hence, their boot time is
25typically much shorter.
26
27
28Configuration
29-------------
30
31### Loading binaries into memory ###
32
33U-Boot needs to load not just Xen, the device tree binary, the dom0 kernel and
34ramdisk. It also needs to load the kernel and ramdisk of any additional domains
35to boot. For example if this is the bootcmd for Xen and Dom0:
36
37    tftpb 0x1280000 xen.dtb
38    tftpb 0x0x80000 xen-Image
39    tftpb 0x1400000 xen.ub
40    tftpb 0x9000000 xen-rootfs.cpio.gz.u-boot
41
42    bootm 0x1400000 0x9000000 0x1280000
43
44If we want to add one DomU with Image-DomU as the DomU kernel
45and ramdisk-DomU as DomU ramdisk:
46
47    tftpb 0x1280000 xen.dtb
48    tftpb 0x80000 xen-Image
49    tftpb 0x1400000 xen.ub
50    tftpb 0x9000000 xen-rootfs.cpio.gz.u-boot
51
52    tftpb 0x2000000 Image-DomU
53    tftpb 0x3000000 ramdisk-DomU
54
55    bootm 0x1400000 0x9000000 0x1280000
56
57
58### Device Tree configuration ###
59
60In addition to loading the necessary binaries, we also need to advertise
61the presence of the additional VM and its configuration. It is done via
62device tree adding a node under /chosen as follows:
63
64    domU1 {
65        #address-cells = <1>;
66        #size-cells = <1>;
67        compatible = "xen,domain";
68        memory = <0 0x20000>;
69        cpus = <1>;
70        vpl011;
71
72        module@2000000 {
73            compatible = "multiboot,kernel", "multiboot,module";
74            reg = <0x2000000 0xffffff>;
75            bootargs = "console=ttyAMA0";
76        };
77
78        module@30000000 {
79            compatible = "multiboot,ramdisk", "multiboot,module";
80            reg = <0x3000000 0xffffff>;
81        };
82    };
83
84Where memory is the memory of the VM in KBs, cpus is the number of
85cpus. module@2000000 and module@3000000 advertise where the kernel and
86ramdisk are in memory.
87
88Note: the size specified should exactly match the size of the Kernel/initramfs.
89Otherwise, they may be unusable in Xen (for instance if they are compressed).
90
91See docs/misc/arm/device-tree/booting.txt for more information.
92
93PV Drivers
94----------
95
96It is possible to use PV drivers with dom0less guests with some
97restrictions:
98
99- dom0less domUs that want to use PV drivers support should have the
100  "xen,enhanced" property set under their device tree nodes (see
101  docs/misc/arm/device-tree/booting.txt)
102- a dom0 must be present (or another domain with enough privileges to
103  run the toolstack)
104- after dom0 is booted, the utility "init-dom0less" must be run
105- do not run "init-dom0less" while creating other guests with xl
106
107After the execution of init-dom0less, it is possible to use "xl" to
108hotplug PV drivers to dom0less guests. E.g. xl network-attach domU.
109
110The implementation works as follows:
111- Xen allocates the xenstore event channel for each dom0less domU that
112  has the "xen,enhanced" property, and sets HVM_PARAM_STORE_EVTCHN
113- Xen does *not* allocate the xenstore page and sets HVM_PARAM_STORE_PFN
114  to ~0ULL (invalid)
115- Dom0less domU kernels check that HVM_PARAM_STORE_PFN is set to invalid
116    - Old kernels will continue without xenstore support (Note: some old
117      buggy kernels might crash because they don't check the validity of
118      HVM_PARAM_STORE_PFN before using it! Disable "xen,enhanced" in
119      those cases)
120    - New kernels will wait for a notification on the xenstore event
121      channel (HVM_PARAM_STORE_EVTCHN) before continuing with the
122      initialization
123- Once dom0 is booted, init-dom0less is executed:
124    - it allocates the xenstore shared page and sets HVM_PARAM_STORE_PFN
125    - it calls xs_introduce_domain
126- Xenstored notices the new domain, initializes interfaces as usual, and
127  sends an event channel notification to the domain using the xenstore
128  event channel (HVM_PARAM_STORE_EVTCHN)
129- The Linux domU kernel receives the event channel notification, checks
130  HVM_PARAM_STORE_PFN again and continue with the initialization
131
132
133Limitations
134-----------
135
136Domains started by Xen at boot time currently have the following
137limitations:
138
139- They cannot be properly shutdown or rebooted using xl. If one of them
140  crashes, the whole platform should be rebooted.
141
142- Some xl operations might not work as expected. xl is meant to be used
143  with domains that have been created by it. Using xl with domains
144  started by Xen at boot might not work as expected.
145
146- The GIC version is the native version. In absence of other
147  information, the GIC version exposed to the domains started by Xen at
148  boot is the same as the native GIC version.
149
150- Pinning vCPUs of domains started by Xen at boot can be
151  done from the control domain, using `xl vcpu-pin` as usual. It is not
152  currently possible to configure vCPU pinning without a control domain.
153  However, the NULL scheduler can be selected by passing `sched=null` to
154  the Xen command line. The NULL scheduler automatically assigns and
155  pins vCPUs to pCPUs, but the vCPU-pCPU assignments cannot be
156  configured.
157
158Notes
159-----
160
161- 'xl console' command will not attach to the domain's console in case
162  of dom0less. DomU are domains created by Xen (similar to Dom0) and
163  therefore they are all managed by Xen and some of the commands may not work.
164
165  A user is allowed to configure the key sequence to switch input.
166  Pressing the Xen "conswitch" (Ctrl-A by default) three times
167  switches input in case of dom0less mode.
168
169- Domains created by Xen will have no name at boot. Domain-0 has a name
170  thanks to the helper xen-init-dom0 called at boot by the initscript.
171  If you want to setup DomU name, then you will have to create the xenstore
172  node associated. By default DomU names are shown as '(null)' in the
173  xl domains list.
174