Re: [Qemu-devel] [PATCH v6 2/7] docs: VM Generation ID device description

[Laszlo Ersek]

On 02/15/17 12:07, Igor Mammedov wrote:
> On Tue, 14 Feb 2017 22:15:44 -0800
> address@hidden wrote:
> 
>> From: Ben Warren <address@hidden>
>>
>> This patch is based off an earlier version by
>> Gal Hammer (address@hidden)
>>
>> Requirements section, ASCII diagrams and overall help
>> provided by Laszlo Ersek (address@hidden)
>>
>> Signed-off-by: Gal Hammer <address@hidden>
>> Signed-off-by: Ben Warren <address@hidden>
>> Reviewed-by: Laszlo Ersek <address@hidden>
> Reviewed-by: Igor Mammedov <address@hidden>

I meticulously compared this version against the docs patch in v5, and I
agree that it's a good update.

I also see that

  [PATCH v6 4/7] ACPI: Add Virtual Machine Generation ID support

adds a reference to this file, as a comment, which was requested by Igor.

Looks good! My R-b stands.

Thank you,
Laszlo

>> ---
>>  docs/specs/vmgenid.txt | 245 
>> +++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 245 insertions(+)
>>  create mode 100644 docs/specs/vmgenid.txt
>>
>> diff --git a/docs/specs/vmgenid.txt b/docs/specs/vmgenid.txt
>> new file mode 100644
>> index 0000000..aa9f518
>> --- /dev/null
>> +++ b/docs/specs/vmgenid.txt
>> @@ -0,0 +1,245 @@
>> +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.
>