Re: [PATCH v7 01/10] ACPI ERST: specification for ERST support

[Ani Sinha]

On Thu, 7 Oct 2021, Eric DeVolder wrote:

> Information on the implementation of the ACPI ERST support.
>
> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>

Acked-by: Ani Sinha <ani@anisinha.ca>

> ---
>  docs/specs/acpi_erst.rst | 200 
> +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 200 insertions(+)
>  create mode 100644 docs/specs/acpi_erst.rst
>
> diff --git a/docs/specs/acpi_erst.rst b/docs/specs/acpi_erst.rst
> new file mode 100644
> index 0000000..a8a9d22
> --- /dev/null
> +++ b/docs/specs/acpi_erst.rst
> @@ -0,0 +1,200 @@
> +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".
> --
> 1.8.3.1
>
>