[RFC v2] ARM VM System Specification

Christoffer Dall christoffer.dall at linaro.org
Fri Mar 28 14:45:17 EDT 2014 

ARM VM System Specification
===========================

Goal
----
The goal of this spec is to allow suitably-built OS images to run on
all ARM virtualization solutions, such as KVM or Xen.

Recommendations in this spec are valid for aarch32 and aarch64 alike, and
they aim to be hypervisor agnostic.

Note that simply adhering to the SBSA [2] is not a valid approach, for
example because the SBSA mandates EL2, which will not be available for
VMs.  Further, this spec also covers the aarch32 execution mode, not
covered in the SBSA.


Image format
------------
The image format, as presented to the VM, needs to be well-defined in
order for prepared disk images to be bootable across various
virtualization implementations.

The raw disk format as presented to the VM must be partitioned with a
GUID Partition Table (GPT).  The bootable software must be placed in the
EFI System Partition (ESP), using the UEFI removable media path, and
must be an EFI application complying to the UEFI Specification 2.4
Revision A [6].

The ESP partition's GPT entry's partition type GUID must be
C12A7328-F81F-11D2-BA4B-00A0C93EC93B and the file system must be
formatted as FAT32/vfat as per Section 12.3.1.1 in [6].

The removable media path is \EFI\BOOT\BOOTARM.EFI for the aarch32
execution state and is \EFI\BOOT\BOOTAA64.EFI for the aarch64 execution
state as specified in Section 3.3 (3.3 (Boot Option Variables Default Boot
Behavior) and 3.4.1.1 (Removable Media Boot Behavior) in [6].

This ensures that tools for both Xen and KVM can load a binary UEFI
firmware which can read and boot the EFI application in the disk image.

A typical scenario will be GRUB2 packaged as an EFI application, which
mounts the system boot partition and boots Linux.


Virtual Firmware
----------------
The VM system must be UEFI compliant in order to be able to boot the EFI
application in the ESP.  It is recommended that this is achieved by
loading a UEFI binary as the first software executed by the VM, which
then executes the EFI application.  The UEFI implementation should be
compliant with UEFI Specification 2.4 Revision A [6] or later.

This document strongly recommends that the VM implementation supports
persistent environment storage for virtual firmware implementation in
order to ensure probable use cases such as adding additional disk images
to a VM or running installers to perform upgrades.

This document strongly recommends that VM implementations implement
persistent variable storage for their UEFI implementation.  Persistent
variable storage shall be a property of a VM instance, but shall not be
stored as part of a portable disk image.  Portable disk images shall
conform to the UEFI removable disk requirements from the UEFI spec and
cannot rely on on a pre-configured UEFI environment.

The binary UEFI firmware implementation should not be distributed as
part of the VM image, but is specific to the VM implementation.


Hardware Description
--------------------
The VM system must be UEFI compliant and therefore the UEFI system table
will provide a means to access hardware description data.

The VM implementation must provide through its UEFI implementation:

  a complete FDT which describes the entire VM system and will boot
  mainline kernels driven by device tree alone

For more information about the arm and arm64 boot conventions, see
Documentation/arm/Booting and Documentation/arm64/booting.txt in the
Linux kernel source tree.

For more information about UEFI booting, see [4] and [5].


VM Platform
-----------
The specification does not mandate any specific memory map.  The guest
OS must be able to enumerate all processing elements, devices, and
memory through HW description data (FDT) or a bus-specific
mechanism such as PCI.

If aarch64 physical CPUs implement support for the aarch32 execution
state in EL1 and EL0 execution, it is recommended that the VM
implementation supports booting the VM at EL1 in both aarch32 and
aarch64 execution states.

The virtual hardware platform must provide a number of mandatory
peripherals:

  Serial console:  The platform should provide a console,
  based on an emulated pl011, a virtio-console, or a Xen PV console.

  An ARM Generic Interrupt Controller v2 (GICv2) [3] or newer.  GICv2
  limits the the number of virtual CPUs to 8 cores, newer GIC versions
  removes this limitation.

  The ARM virtual timer and counter should be available to the VM as
  per the ARM Generic Timers specification in the ARM ARM [1].

It is strongly recommended that the VM implementation provides a
hotpluggable bus to support hotplug of at least block and network
devices.  Suitable buses include a virtual PCIe bus and the Xen PV bus.

For the VM image to be compliant with this spec, the following applies
for the guest OS in the VM image:

  The guest OS must include support for pl011 UART, virtio-console, and
  the Xen PV console.

  The guest OS must include support for GICv2 and any available newer
  version of the GIC architecture to maintain compatibility with older
  VM implementations.

  It is strongly recommended to include support for all available
  (block, network, console, balloon) virtio-pci, virtio-mmio, and Xen PV
  drivers in the guest OS kernel or initial ramdisk.


Other common peripherals for block devices, networking, and more can
(and typically will) be provided, but OS software written and compiled
to run on ARM VMs cannot make any assumptions about which variations
of these should exist or which implementation they use (e.g. VirtIO or
Xen PV).  See "Hardware Description" above.


Changes from previous versions of this RFC
------------------------------------------
Changes v1-v2:
 - Clearly specify that the guest must support the pl011,
   virtio-console, and Xen PV console.  (Note that it was discussed to
   mandate a pl011 during Linaro Connect Asia 2014, but that was under the
   impression that the SBSA specification was an output-only no-irq
   serial port, which is not the case.  The only two benefits from
   mandating a specific serial type was to handle "console=ttyAMA0"
   kernel command line parameters and earlyprintk; Grant Likely has
   submitted patches to avoid the need for "console=" parameters, and
   Rob Herring has submitted patches for paravirtualized earlyprintk
   consoles.)
 - Reference EFI specification for bootable paths.
 - Remove discussion on ACPI boot methods and do not suggest ACPI
   support in VMs.
 - Add specification about UEFI persistent variable storage and
   portability.


References
----------
[1]: The ARM Architecture Reference Manual, ARMv8, Issue A.b
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0487a.b/index.html

[2]: ARM Server Base System Architecture
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.den0029/index.html

[3]: The ARM Generic Interrupt Controller Architecture Specifications v2.0
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0487a.b/index.html

[4]: http://www.secretlab.ca/archives/27

[5]: https://git.linaro.org/people/leif.lindholm/linux.git/blob/refs/heads/uefi-for-upstream:/Documentation/arm/uefi.txt

[6]: UEFI Specification 2.4 Revision A
http://www.uefi.org/sites/default/files/resources/2_4_Errata_A.pdf

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