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