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Re: [Qemu-devel] [PATCH v6 2/7] docs: VM Generation ID device descriptio
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH v6 2/7] docs: VM Generation ID device description |
Date: |
Wed, 15 Feb 2017 15:26:10 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1 |
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.
>
[Qemu-devel] [PATCH v6 7/7] tests: Add unit tests for the VM Generation ID feature, ben, 2017/02/15