247 lines
8.8 KiB
ReStructuredText
247 lines
8.8 KiB
ReStructuredText
|
Virtual Machine Generation ID Device
|
||
|
|
====================================
|
||
|
|
|
||
|
|
..
|
||
|
|
Copyright (C) 2016 Red Hat, Inc.
|
||
|
|
Copyright (C) 2017 Skyport Systems, Inc.
|
||
|
|
|
||
|
|
This work is licensed under the terms of the GNU GPL, version 2 or later.
|
||
|
|
See the COPYING file in the top-level directory.
|
||
|
|
|
||
|
|
The VM generation ID (``vmgenid``) device is an emulated device which
|
||
|
|
exposes a 128-bit, cryptographically random, integer value identifier,
|
||
|
|
referred to as a Globally Unique Identifier, or GUID.
|
||
|
|
|
||
|
|
This allows management applications (e.g. libvirt) to notify the guest
|
||
|
|
operating system when the virtual machine is executed with a different
|
||
|
|
configuration (e.g. snapshot execution or creation from a template). The
|
||
|
|
guest operating system notices the change, and is then able to react as
|
||
|
|
appropriate by marking its copies of distributed databases as dirty,
|
||
|
|
re-initializing its random number generator etc.
|
||
|
|
|
||
|
|
|
||
|
|
Requirements
|
||
|
|
------------
|
||
|
|
|
||
|
|
These requirements are extracted from the "How to implement virtual machine
|
||
|
|
generation ID support in a virtualization platform" section of
|
||
|
|
`the Microsoft Virtual Machine Generation ID specification
|
||
|
|
<http://go.microsoft.com/fwlink/?LinkId=260709>`_ dated August 1, 2012.
|
||
|
|
|
||
|
|
- **R1a** The generation ID shall live in an 8-byte aligned buffer.
|
||
|
|
|
||
|
|
- **R1b** The buffer holding the generation ID shall be in guest RAM,
|
||
|
|
ROM, or device MMIO range.
|
||
|
|
|
||
|
|
- **R1c** The buffer holding the generation ID shall be kept separate from
|
||
|
|
areas used by the operating system.
|
||
|
|
|
||
|
|
- **R1d** The buffer shall not be covered by an AddressRangeMemory or
|
||
|
|
AddressRangeACPI entry in the E820 or UEFI memory map.
|
||
|
|
|
||
|
|
- **R1e** The generation ID shall not live in a page frame that could be
|
||
|
|
mapped with caching disabled. (In other words, regardless of whether the
|
||
|
|
generation ID lives in RAM, ROM or MMIO, it shall only be mapped as
|
||
|
|
cacheable.)
|
||
|
|
|
||
|
|
- **R2** to **R5** [These AML requirements are isolated well enough in the
|
||
|
|
Microsoft specification for us to simply refer to them here.]
|
||
|
|
|
||
|
|
- **R6** The hypervisor shall expose a _HID (hardware identifier) object
|
||
|
|
in the VMGenId device's scope that is unique to the hypervisor vendor.
|
||
|
|
|
||
|
|
|
||
|
|
QEMU Implementation
|
||
|
|
-------------------
|
||
|
|
|
||
|
|
The above-mentioned specification does not dictate which ACPI descriptor table
|
||
|
|
will contain the VM Generation ID device. Other implementations (Hyper-V and
|
||
|
|
Xen) put it in the main descriptor table (Differentiated System Description
|
||
|
|
Table or DSDT). For ease of debugging and implementation, we have decided to
|
||
|
|
put it in its own Secondary System Description Table, or SSDT.
|
||
|
|
|
||
|
|
The following is a dump of the contents from a running system::
|
||
|
|
|
||
|
|
# iasl -p ./SSDT -d /sys/firmware/acpi/tables/SSDT
|
||
|
|
|
||
|
|
Intel ACPI Component Architecture
|
||
|
|
ASL+ Optimizing Compiler version 20150717-64
|
||
|
|
Copyright (c) 2000 - 2015 Intel Corporation
|
||
|
|
|
||
|
|
Reading ACPI table from file /sys/firmware/acpi/tables/SSDT - Length
|
||
|
|
00000198 (0x0000C6)
|
||
|
|
ACPI: SSDT 0x0000000000000000 0000C6 (v01 BOCHS VMGENID 00000001 BXPC 00000001)
|
||
|
|
Acpi table [SSDT] successfully installed and loaded
|
||
|
|
Pass 1 parse of [SSDT]
|
||
|
|
Pass 2 parse of [SSDT]
|
||
|
|
Parsing Deferred Opcodes (Methods/Buffers/Packages/Regions)
|
||
|
|
|
||
|
|
Parsing completed
|
||
|
|
Disassembly completed
|
||
|
|
ASL Output: ./SSDT.dsl - 1631 bytes
|
||
|
|
# cat SSDT.dsl
|
||
|
|
/*
|
||
|
|
* Intel ACPI Component Architecture
|
||
|
|
* AML/ASL+ Disassembler version 20150717-64
|
||
|
|
* Copyright (c) 2000 - 2015 Intel Corporation
|
||
|
|
*
|
||
|
|
* Disassembling to symbolic ASL+ operators
|
||
|
|
*
|
||
|
|
* Disassembly of /sys/firmware/acpi/tables/SSDT, Sun Feb 5 00:19:37 2017
|
||
|
|
*
|
||
|
|
* Original Table Header:
|
||
|
|
* Signature "SSDT"
|
||
|
|
* Length 0x000000CA (202)
|
||
|
|
* Revision 0x01
|
||
|
|
* Checksum 0x4B
|
||
|
|
* OEM ID "BOCHS "
|
||
|
|
* OEM Table ID "VMGENID"
|
||
|
|
* OEM Revision 0x00000001 (1)
|
||
|
|
* Compiler ID "BXPC"
|
||
|
|
* Compiler Version 0x00000001 (1)
|
||
|
|
*/
|
||
|
|
DefinitionBlock ("/sys/firmware/acpi/tables/SSDT.aml", "SSDT", 1, "BOCHS ", "VMGENID", 0x00000001)
|
||
|
|
{
|
||
|
|
Name (VGIA, 0x07FFF000)
|
||
|
|
Scope (\_SB)
|
||
|
|
{
|
||
|
|
Device (VGEN)
|
||
|
|
{
|
||
|
|
Name (_HID, "QEMUVGID") // _HID: Hardware ID
|
||
|
|
Name (_CID, "VM_Gen_Counter") // _CID: Compatible ID
|
||
|
|
Name (_DDN, "VM_Gen_Counter") // _DDN: DOS Device Name
|
||
|
|
Method (_STA, 0, NotSerialized) // _STA: Status
|
||
|
|
{
|
||
|
|
Local0 = 0x0F
|
||
|
|
If ((VGIA == Zero))
|
||
|
|
{
|
||
|
|
Local0 = Zero
|
||
|
|
}
|
||
|
|
|
||
|
|
Return (Local0)
|
||
|
|
}
|
||
|
|
|
||
|
|
Method (ADDR, 0, NotSerialized)
|
||
|
|
{
|
||
|
|
Local0 = Package (0x02) {}
|
||
|
|
Index (Local0, Zero) = (VGIA + 0x28)
|
||
|
|
Index (Local0, One) = Zero
|
||
|
|
Return (Local0)
|
||
|
|
}
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
Method (\_GPE._E05, 0, NotSerialized) // _Exx: Edge-Triggered GPE
|
||
|
|
{
|
||
|
|
Notify (\_SB.VGEN, 0x80) // Status Change
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
|
||
|
|
Design Details:
|
||
|
|
---------------
|
||
|
|
|
||
|
|
Requirements R1a through R1e dictate that the memory holding the
|
||
|
|
VM Generation ID must be allocated and owned by the guest firmware,
|
||
|
|
in this case BIOS or UEFI. However, to be useful, QEMU must be able to
|
||
|
|
change the contents of the memory at runtime, specifically when starting a
|
||
|
|
backed-up or snapshotted image. In order to do this, QEMU must know the
|
||
|
|
address that has been allocated.
|
||
|
|
|
||
|
|
The mechanism chosen for this memory sharing is writable fw_cfg blobs.
|
||
|
|
These are data object that are visible to both QEMU and guests, and are
|
||
|
|
addressable as sequential files.
|
||
|
|
|
||
|
|
More information about fw_cfg can be found in :doc:`fw_cfg`.
|
||
|
|
|
||
|
|
Two fw_cfg blobs are used in this case:
|
||
|
|
|
||
|
|
``/etc/vmgenid_guid``
|
||
|
|
|
||
|
|
- contains the actual VM Generation ID GUID
|
||
|
|
- read-only to the guest
|
||
|
|
|
||
|
|
``/etc/vmgenid_addr``
|
||
|
|
|
||
|
|
- contains the address of the downloaded vmgenid blob
|
||
|
|
- writable by the guest
|
||
|
|
|
||
|
|
|
||
|
|
QEMU sends the following commands to the guest at startup:
|
||
|
|
|
||
|
|
1. Allocate memory for vmgenid_guid fw_cfg blob.
|
||
|
|
2. Write the address of vmgenid_guid into the SSDT (VGIA ACPI variable as
|
||
|
|
shown above in the iasl dump). Note that this change is not propagated
|
||
|
|
back to QEMU.
|
||
|
|
3. Write the address of vmgenid_guid back to QEMU's copy of vmgenid_addr
|
||
|
|
via the fw_cfg DMA interface.
|
||
|
|
|
||
|
|
After step 3, QEMU is able to update the contents of vmgenid_guid at will.
|
||
|
|
|
||
|
|
Since BIOS or UEFI does not necessarily run when we wish to change the GUID,
|
||
|
|
the value of VGIA is persisted via the VMState mechanism.
|
||
|
|
|
||
|
|
As spelled out in the specification, any change to the GUID executes an
|
||
|
|
ACPI notification. The exact handler to use is not specified, so the vmgenid
|
||
|
|
device uses the first unused one: ``\_GPE._E05``.
|
||
|
|
|
||
|
|
|
||
|
|
Endian-ness Considerations:
|
||
|
|
---------------------------
|
||
|
|
|
||
|
|
Although not specified in Microsoft's document, it is assumed that the
|
||
|
|
device is expected to use little-endian format.
|
||
|
|
|
||
|
|
All GUID passed in via command line or monitor are treated as big-endian.
|
||
|
|
GUID values displayed via monitor are shown in big-endian format.
|
||
|
|
|
||
|
|
|
||
|
|
GUID Storage Format:
|
||
|
|
--------------------
|
||
|
|
|
||
|
|
In order to implement an OVMF "SDT Header Probe Suppressor", the contents of
|
||
|
|
the vmgenid_guid fw_cfg blob are not simply a 128-bit GUID. There is also
|
||
|
|
significant padding in order to align and fill a memory page, as shown in the
|
||
|
|
following diagram::
|
||
|
|
|
||
|
|
+----------------------------------+
|
||
|
|
| SSDT with OEM Table ID = VMGENID |
|
||
|
|
+----------------------------------+
|
||
|
|
| ... | TOP OF PAGE
|
||
|
|
| VGIA dword object ---------------|-----> +---------------------------+
|
||
|
|
| ... | | fw-allocated array for |
|
||
|
|
| _STA method referring to VGIA | | "etc/vmgenid_guid" |
|
||
|
|
| ... | +---------------------------+
|
||
|
|
| ADDR method referring to VGIA | | 0: OVMF SDT Header probe |
|
||
|
|
| ... | | suppressor |
|
||
|
|
+----------------------------------+ | 36: padding for 8-byte |
|
||
|
|
| alignment |
|
||
|
|
| 40: GUID |
|
||
|
|
| 56: padding to page size |
|
||
|
|
+---------------------------+
|
||
|
|
END OF PAGE
|
||
|
|
|
||
|
|
|
||
|
|
Device Usage:
|
||
|
|
-------------
|
||
|
|
|
||
|
|
The device has one property, which may be only be set using the command line:
|
||
|
|
|
||
|
|
``guid``
|
||
|
|
sets the value of the GUID. A special value ``auto`` instructs
|
||
|
|
QEMU to generate a new random GUID.
|
||
|
|
|
||
|
|
For example::
|
||
|
|
|
||
|
|
QEMU -device vmgenid,guid="324e6eaf-d1d1-4bf6-bf41-b9bb6c91fb87"
|
||
|
|
QEMU -device vmgenid,guid=auto
|
||
|
|
|
||
|
|
The property may be queried via QMP/HMP::
|
||
|
|
|
||
|
|
(QEMU) query-vm-generation-id
|
||
|
|
{"return": {"guid": "324e6eaf-d1d1-4bf6-bf41-b9bb6c91fb87"}}
|
||
|
|
|
||
|
|
Setting of this parameter is intentionally left out from the QMP/HMP
|
||
|
|
interfaces. There are no known use cases for changing the GUID once QEMU is
|
||
|
|
running, and adding this capability would greatly increase the complexity.
|