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+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".
