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diff --git a/docs/specs/vmgenid.rst b/docs/specs/vmgenid.rst new file mode 100644 index 0000000..9a3cefc --- /dev/null +++ b/docs/specs/vmgenid.rst @@ -0,0 +1,246 @@ +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. |