docs/specs/acpi_erst.rst - qemu - Git at Google

 ACPI ERST DEVICE
 ================

 The ACPI ERST device is utilized to support the ACPI Error Record
 Serialization Table, ERST, functionality. This feature is designed for
 storing error records in persistent storage for future reference
 and/or debugging.

 The ACPI specification[1], in Chapter "ACPI Platform Error Interfaces
 (APEI)", and specifically subsection "Error Serialization", outlines a
 method for storing error records into persistent storage.

 The format of error records is described in the UEFI specification[2],
 in Appendix N "Common Platform Error Record".

 While the ACPI specification allows for an NVRAM "mode" (see
 GET_ERROR_LOG_ADDRESS_RANGE_ATTRIBUTES) where non-volatile RAM is
 directly exposed for direct access by the OS/guest, this device
 implements the non-NVRAM "mode". This non-NVRAM "mode" is what is
 implemented by most BIOS (since flash memory requires programming
 operations in order to update its contents). Furthermore, as of the
 time of this writing, Linux only supports the non-NVRAM "mode".


 Background/Motivation
 ---------------------

 Linux uses the persistent storage filesystem, pstore, to record
 information (eg. dmesg tail) upon panics and shutdowns.  Pstore is
 independent of, and runs before, kdump.  In certain scenarios (ie.
 hosts/guests with root filesystems on NFS/iSCSI where networking
 software and/or hardware fails, and thus kdump fails), pstore may
 contain information available for post-mortem debugging.

 Two common storage backends for the pstore filesystem are ACPI ERST
 and UEFI. Most BIOS implement ACPI ERST. UEFI is not utilized in all
 guests. With QEMU supporting ACPI ERST, it becomes a viable pstore
 storage backend for virtual machines (as it is now for bare metal
 machines).

 Enabling support for ACPI ERST facilitates a consistent method to
 capture kernel panic information in a wide range of guests: from
 resource-constrained microvms to very large guests, and in particular,
 in direct-boot environments (which would lack UEFI run-time services).

 Note that Microsoft Windows also utilizes the ACPI ERST for certain
 crash information, if available[3].


 Configuration|Usage
 -------------------

 To use ACPI ERST, a memory-backend-file object and acpi-erst device
 can be created, for example:

  qemu ...
  -object memory-backend-file,id=erstnvram,mem-path=acpi-erst.backing,size=0x10000,share=on \
  -device acpi-erst,memdev=erstnvram

 For proper operation, the ACPI ERST device needs a memory-backend-file
 object with the following parameters:

  - id: The id of the memory-backend-file object is used to associate
    this memory with the acpi-erst device.
  - size: The size of the ACPI ERST backing storage. This parameter is
    required.
  - mem-path: The location of the ACPI ERST backing storage file. This
    parameter is also required.
  - share: The share=on parameter is required so that updates to the
    ERST backing store are written to the file.

 and ERST device:

  - memdev: Is the object id of the memory-backend-file.
  - record_size: Specifies the size of the records (or slots) in the
    backend storage. Must be a power of two value greater than or
    equal to 4096 (PAGE_SIZE).


 PCI Interface
 -------------

 The ERST device is a PCI device with two BARs, one for accessing the
 programming registers, and the other for accessing the record exchange
 buffer.

 BAR0 contains the programming interface consisting of ACTION and VALUE
 64-bit registers.  All ERST actions/operations/side effects happen on
 the write to the ACTION, by design. Any data needed by the action must
 be placed into VALUE prior to writing ACTION.  Reading the VALUE
 simply returns the register contents, which can be updated by a
 previous ACTION.

 BAR1 contains the 8KiB record exchange buffer, which is the
 implemented maximum record size.


 Backend Storage Format
 ----------------------

 The backend storage is divided into fixed size "slots", 8KiB in
 length, with each slot storing a single record.  Not all slots need to
 be occupied, and they need not be occupied in a contiguous fashion.
 The ability to clear/erase specific records allows for the formation
 of unoccupied slots.

 Slot 0 contains a backend storage header that identifies the contents
 as ERST and also facilitates efficient access to the records.
 Depending upon the size of the backend storage, additional slots will
 be designated to be a part of the slot 0 header. For example, at 8KiB,
 the slot 0 header can accomodate 1021 records. Thus a storage size
 of 8MiB (8KiB * 1024) requires an additional slot for use by the
 header. In this scenario, slot 0 and slot 1 form the backend storage
 header, and records can be stored starting at slot 2.

 Below is an example layout of the backend storage format (for storage
 size less than 8MiB). The size of the storage is a multiple of 8KiB,
 and contains N number of slots to store records. The example below
 shows two records (in CPER format) in the backend storage, while the
 remaining slots are empty/available.

 ::

  Slot   Record
         <------------------ 8KiB -------------------->
         +--------------------------------------------+
     0   | storage header                             |
         +--------------------------------------------+
     1   | empty/available                            |
         +--------------------------------------------+
     2   | CPER                                       |
         +--------------------------------------------+
     3   | CPER                                       |
         +--------------------------------------------+
   ...   |                                            |
         +--------------------------------------------+
     N   | empty/available                            |
         +--------------------------------------------+

 The storage header consists of some basic information and an array
 of CPER record_id's to efficiently access records in the backend
 storage.

 All fields in the header are stored in little endian format.

 ::

   +--------------------------------------------+
   | magic                                      | 0x0000
   +--------------------------------------------+
   | record_offset        | record_size         | 0x0008
   +--------------------------------------------+
   | record_count         | reserved | version  | 0x0010
   +--------------------------------------------+
   | record_id[0]                               | 0x0018
   +--------------------------------------------+
   | record_id[1]                               | 0x0020
   +--------------------------------------------+
   | record_id[...]                             |
   +--------------------------------------------+
   | record_id[N]                               | 0x1FF8
   +--------------------------------------------+

 The 'magic' field contains the value 0x524F545354535245.

 The 'record_size' field contains the value 0x2000, 8KiB.

 The 'record_offset' field points to the first record_id in the array,
 0x0018.

 The 'version' field contains 0x0100, the first version.

 The 'record_count' field contains the number of valid records in the
 backend storage.

 The 'record_id' array fields are the 64-bit record identifiers of the
 CPER record in the corresponding slot. Stated differently, the
 location of a CPER record_id in the record_id[] array provides the
 slot index for the corresponding record in the backend storage.

 Note that, for example, with a backend storage less than 8MiB, slot 0
 contains the header, so the record_id[0] will never contain a valid
 CPER record_id. Instead slot 1 is the first available slot and thus
 record_id_[1] may contain a CPER.

 A 'record_id' of all 0s or all 1s indicates an invalid record (ie. the
 slot is available).


 References
 ----------

 [1] "Advanced Configuration and Power Interface Specification",
     version 4.0, June 2009.

 [2] "Unified Extensible Firmware Interface Specification",
     version 2.1, October 2008.

 [3] "Windows Hardware Error Architecture", specfically
     "Error Record Persistence Mechanism".
	ACPI ERST DEVICE
	================

	The ACPI ERST device is utilized to support the ACPI Error Record
	Serialization Table, ERST, functionality. This feature is designed for
	storing error records in persistent storage for future reference
	and/or debugging.

	The ACPI specification[1], in Chapter "ACPI Platform Error Interfaces
	(APEI)", and specifically subsection "Error Serialization", outlines a
	method for storing error records into persistent storage.

	The format of error records is described in the UEFI specification[2],
	in Appendix N "Common Platform Error Record".

	While the ACPI specification allows for an NVRAM "mode" (see
	GET_ERROR_LOG_ADDRESS_RANGE_ATTRIBUTES) where non-volatile RAM is
	directly exposed for direct access by the OS/guest, this device
	implements the non-NVRAM "mode". This non-NVRAM "mode" is what is
	implemented by most BIOS (since flash memory requires programming
	operations in order to update its contents). Furthermore, as of the
	time of this writing, Linux only supports the non-NVRAM "mode".


	Background/Motivation
	---------------------

	Linux uses the persistent storage filesystem, pstore, to record
	information (eg. dmesg tail) upon panics and shutdowns. Pstore is
	independent of, and runs before, kdump. In certain scenarios (ie.
	hosts/guests with root filesystems on NFS/iSCSI where networking
	software and/or hardware fails, and thus kdump fails), pstore may
	contain information available for post-mortem debugging.

	Two common storage backends for the pstore filesystem are ACPI ERST
	and UEFI. Most BIOS implement ACPI ERST. UEFI is not utilized in all
	guests. With QEMU supporting ACPI ERST, it becomes a viable pstore
	storage backend for virtual machines (as it is now for bare metal
	machines).

	Enabling support for ACPI ERST facilitates a consistent method to
	capture kernel panic information in a wide range of guests: from
	resource-constrained microvms to very large guests, and in particular,
	in direct-boot environments (which would lack UEFI run-time services).

	Note that Microsoft Windows also utilizes the ACPI ERST for certain
	crash information, if available[3].


	Configuration\|Usage
	-------------------

	To use ACPI ERST, a memory-backend-file object and acpi-erst device
	can be created, for example:

	qemu ...
	-object memory-backend-file,id=erstnvram,mem-path=acpi-erst.backing,size=0x10000,share=on \
	-device acpi-erst,memdev=erstnvram

	For proper operation, the ACPI ERST device needs a memory-backend-file
	object with the following parameters:

	- id: The id of the memory-backend-file object is used to associate
	this memory with the acpi-erst device.
	- size: The size of the ACPI ERST backing storage. This parameter is
	required.
	- mem-path: The location of the ACPI ERST backing storage file. This
	parameter is also required.
	- share: The share=on parameter is required so that updates to the
	ERST backing store are written to the file.

	and ERST device:

	- memdev: Is the object id of the memory-backend-file.
	- record_size: Specifies the size of the records (or slots) in the
	backend storage. Must be a power of two value greater than or
	equal to 4096 (PAGE_SIZE).


	PCI Interface
	-------------

	The ERST device is a PCI device with two BARs, one for accessing the
	programming registers, and the other for accessing the record exchange
	buffer.

	BAR0 contains the programming interface consisting of ACTION and VALUE
	64-bit registers. All ERST actions/operations/side effects happen on
	the write to the ACTION, by design. Any data needed by the action must
	be placed into VALUE prior to writing ACTION. Reading the VALUE
	simply returns the register contents, which can be updated by a
	previous ACTION.

	BAR1 contains the 8KiB record exchange buffer, which is the
	implemented maximum record size.


	Backend Storage Format
	----------------------

	The backend storage is divided into fixed size "slots", 8KiB in
	length, with each slot storing a single record. Not all slots need to
	be occupied, and they need not be occupied in a contiguous fashion.
	The ability to clear/erase specific records allows for the formation
	of unoccupied slots.

	Slot 0 contains a backend storage header that identifies the contents
	as ERST and also facilitates efficient access to the records.
	Depending upon the size of the backend storage, additional slots will
	be designated to be a part of the slot 0 header. For example, at 8KiB,
	the slot 0 header can accomodate 1021 records. Thus a storage size
	of 8MiB (8KiB * 1024) requires an additional slot for use by the
	header. In this scenario, slot 0 and slot 1 form the backend storage
	header, and records can be stored starting at slot 2.

	Below is an example layout of the backend storage format (for storage
	size less than 8MiB). The size of the storage is a multiple of 8KiB,
	and contains N number of slots to store records. The example below
	shows two records (in CPER format) in the backend storage, while the
	remaining slots are empty/available.

	::

	Slot Record
	<------------------ 8KiB -------------------->
	+--------------------------------------------+
	0 \| storage header \|
	+--------------------------------------------+
	1 \| empty/available \|
	+--------------------------------------------+
	2 \| CPER \|
	+--------------------------------------------+
	3 \| CPER \|
	+--------------------------------------------+
	... \| \|
	+--------------------------------------------+
	N \| empty/available \|
	+--------------------------------------------+

	The storage header consists of some basic information and an array
	of CPER record_id's to efficiently access records in the backend
	storage.

	All fields in the header are stored in little endian format.

	::

	+--------------------------------------------+
	\| magic \| 0x0000
	+--------------------------------------------+
	\| record_offset \| record_size \| 0x0008
	+--------------------------------------------+
	\| record_count \| reserved \| version \| 0x0010
	+--------------------------------------------+
	\| record_id[0] \| 0x0018
	+--------------------------------------------+
	\| record_id[1] \| 0x0020
	+--------------------------------------------+
	\| record_id[...] \|
	+--------------------------------------------+
	\| record_id[N] \| 0x1FF8
	+--------------------------------------------+

	The 'magic' field contains the value 0x524F545354535245.

	The 'record_size' field contains the value 0x2000, 8KiB.

	The 'record_offset' field points to the first record_id in the array,
	0x0018.

	The 'version' field contains 0x0100, the first version.

	The 'record_count' field contains the number of valid records in the
	backend storage.

	The 'record_id' array fields are the 64-bit record identifiers of the
	CPER record in the corresponding slot. Stated differently, the
	location of a CPER record_id in the record_id[] array provides the
	slot index for the corresponding record in the backend storage.

	Note that, for example, with a backend storage less than 8MiB, slot 0
	contains the header, so the record_id[0] will never contain a valid
	CPER record_id. Instead slot 1 is the first available slot and thus
	record_id_[1] may contain a CPER.

	A 'record_id' of all 0s or all 1s indicates an invalid record (ie. the
	slot is available).


	References
	----------

	[1] "Advanced Configuration and Power Interface Specification",
	version 4.0, June 2009.

	[2] "Unified Extensible Firmware Interface Specification",
	version 2.1, October 2008.

	[3] "Windows Hardware Error Architecture", specfically
	"Error Record Persistence Mechanism".