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Re: [Qemu-devel] [PATCH v5 03/10] docs: VM Generation ID device descript
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH v5 03/10] docs: VM Generation ID device description |
Date: |
Tue, 7 Feb 2017 21:54:46 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 |
On 02/05/17 10:11, 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>
> ---
> docs/specs/vmgenid.txt | 239
> +++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 239 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..d36f8bd
> --- /dev/null
> +++ b/docs/specs/vmgenid.txt
> @@ -0,0 +1,239 @@
> +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 dicate that the memory holding the
> +VM Generation ID must be allocated and owned by the guest operating system,
s/guest operating system/guest firmware/, in my opinion. Do you agree?
> +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 in-memory data structures that are visible to both QEMU and guests,
> +and are addressable as files.
The expression "in-memory data structures" is misleading; they need not
exist in complete (or any) form in guest RAM.
I recommend "data objects" instead (and maybe reference
"docs/specs/fw_cfg.txt").
Also, "addressable as files" is too generic, we don't have random
access. I'd say "addressable as sequential files" (we have rewind (by
way of select), skip, read, and write).
> +
> +Two fw_cfg blobs are used in this case:
> +
> +/etc/vmgenid - contains the actual VM Generation ID GUID
> + - read-only to the guest
> +/etc/vmgenid_addr - contains the address of the GUID
Please replace "address of the GUID" with "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 fw_cfg blob.
> +2. Write the address of vmgenid 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 back to QEMU's copy of vmgenid_addr
> + via the fw_cfg DMA interface.
Please drop "QEMU's copy of".
(All fw_cfg blobs are owned by QEMU, and for some of those, it makes
sense for the firmware to have a copy. In this case, the firmware has no
copy, and QEMU's "copy" is the original thing.)
> +
> +After step 3, QEMU is able to update the contents of vmgenid 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 work with OVMF "SDT Header Probe Supressor", the contents of
s/work with/implement an/
> +the vmgenid 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" |
> +| ... | +---------------------------+
> +| 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
Hahaha, Laszlo cannot multiply. In my previous review I wrote "48" as
start offset for "padding to page size", which implies an 8-byte
(64-bit) GUID... In fact the GUID takes 128 bits, hence 16 bytes. Good
catch! :)
> +
> +
> +Device Usage:
> +-------------
> +
> +The device has one property, which can be set using the command line
> +argument or the QMP interface:
> +
> + 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"
> +
> +Or to change guid in runtime use:
> +
> + set-vm-generation-id guid=124e6eaf-d1d1-4bf6-bf41-b9bb6c91fb87
> + set-vm-generation-id guid=auto
>
I like this version very much. Please fix up the warts identified by
Eric and myself, and then you can add my
Reviewed-by: Laszlo Ersek <address@hidden>
for the next version.
(If you change other parts as well, then please don't add my R-b.)
Thank you!
Laszlo
- [Qemu-devel] [PATCH v5 01/10] ACPI: Add a function for building named qword entries, (continued)
[Qemu-devel] [PATCH v5 04/10] ACPI: Add vmgenid storage entries to the build tables, ben, 2017/02/05
[Qemu-devel] [PATCH v5 03/10] docs: VM Generation ID device description, ben, 2017/02/05
[Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, ben, 2017/02/05
- Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, Michael S. Tsirkin, 2017/02/06
- Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, Ben Warren, 2017/02/06
- Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, Michael S. Tsirkin, 2017/02/06
- Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, Ben Warren, 2017/02/06
- Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, Michael S. Tsirkin, 2017/02/06
- Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, Ben Warren, 2017/02/06
- Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, Michael S. Tsirkin, 2017/02/06
- Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support, Ben Warren, 2017/02/06