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>

1 files changed, 245 insertions, 0 deletions

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+VIRTUAL MACHINE GENERATION ID
+=============================
+
+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
+specification, dated August 1, 2012.
+
+
+The document may be found on the web at:
+  http://go.microsoft.com/fwlink/?LinkId=260709
+
+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 writeable 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 "docs/specs/fw_cfg.txt"
+
+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
+                  - writeable 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.