On 02/05/17 10:11, b...@skyportsystems.com wrote: > From: Ben Warren <b...@skyportsystems.com> > > This patch is based off an earlier version by > Gal Hammer (gham...@redhat.com) > > Requirements section, ASCII diagrams and overall help > provided by Laszlo Ersek (ler...@redhat.com) > > Signed-off-by: Gal Hammer <gham...@redhat.com> > Signed-off-by: Ben Warren <b...@skyportsystems.com> > --- > 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 <ler...@redhat.com> for the next version. (If you change other parts as well, then please don't add my R-b.) Thank you! Laszlo
