acpi_erst.rst (7999B)
1 ACPI ERST DEVICE 2 ================ 3 4 The ACPI ERST device is utilized to support the ACPI Error Record 5 Serialization Table, ERST, functionality. This feature is designed for 6 storing error records in persistent storage for future reference 7 and/or debugging. 8 9 The ACPI specification[1], in Chapter "ACPI Platform Error Interfaces 10 (APEI)", and specifically subsection "Error Serialization", outlines a 11 method for storing error records into persistent storage. 12 13 The format of error records is described in the UEFI specification[2], 14 in Appendix N "Common Platform Error Record". 15 16 While the ACPI specification allows for an NVRAM "mode" (see 17 GET_ERROR_LOG_ADDRESS_RANGE_ATTRIBUTES) where non-volatile RAM is 18 directly exposed for direct access by the OS/guest, this device 19 implements the non-NVRAM "mode". This non-NVRAM "mode" is what is 20 implemented by most BIOS (since flash memory requires programming 21 operations in order to update its contents). Furthermore, as of the 22 time of this writing, Linux only supports the non-NVRAM "mode". 23 24 25 Background/Motivation 26 --------------------- 27 28 Linux uses the persistent storage filesystem, pstore, to record 29 information (eg. dmesg tail) upon panics and shutdowns. Pstore is 30 independent of, and runs before, kdump. In certain scenarios (ie. 31 hosts/guests with root filesystems on NFS/iSCSI where networking 32 software and/or hardware fails, and thus kdump fails), pstore may 33 contain information available for post-mortem debugging. 34 35 Two common storage backends for the pstore filesystem are ACPI ERST 36 and UEFI. Most BIOS implement ACPI ERST. UEFI is not utilized in all 37 guests. With QEMU supporting ACPI ERST, it becomes a viable pstore 38 storage backend for virtual machines (as it is now for bare metal 39 machines). 40 41 Enabling support for ACPI ERST facilitates a consistent method to 42 capture kernel panic information in a wide range of guests: from 43 resource-constrained microvms to very large guests, and in particular, 44 in direct-boot environments (which would lack UEFI run-time services). 45 46 Note that Microsoft Windows also utilizes the ACPI ERST for certain 47 crash information, if available[3]. 48 49 50 Configuration|Usage 51 ------------------- 52 53 To use ACPI ERST, a memory-backend-file object and acpi-erst device 54 can be created, for example: 55 56 qemu ... 57 -object memory-backend-file,id=erstnvram,mem-path=acpi-erst.backing,size=0x10000,share=on \ 58 -device acpi-erst,memdev=erstnvram 59 60 For proper operation, the ACPI ERST device needs a memory-backend-file 61 object with the following parameters: 62 63 - id: The id of the memory-backend-file object is used to associate 64 this memory with the acpi-erst device. 65 - size: The size of the ACPI ERST backing storage. This parameter is 66 required. 67 - mem-path: The location of the ACPI ERST backing storage file. This 68 parameter is also required. 69 - share: The share=on parameter is required so that updates to the 70 ERST backing store are written to the file. 71 72 and ERST device: 73 74 - memdev: Is the object id of the memory-backend-file. 75 - record_size: Specifies the size of the records (or slots) in the 76 backend storage. Must be a power of two value greater than or 77 equal to 4096 (PAGE_SIZE). 78 79 80 PCI Interface 81 ------------- 82 83 The ERST device is a PCI device with two BARs, one for accessing the 84 programming registers, and the other for accessing the record exchange 85 buffer. 86 87 BAR0 contains the programming interface consisting of ACTION and VALUE 88 64-bit registers. All ERST actions/operations/side effects happen on 89 the write to the ACTION, by design. Any data needed by the action must 90 be placed into VALUE prior to writing ACTION. Reading the VALUE 91 simply returns the register contents, which can be updated by a 92 previous ACTION. 93 94 BAR1 contains the 8KiB record exchange buffer, which is the 95 implemented maximum record size. 96 97 98 Backend Storage Format 99 ---------------------- 100 101 The backend storage is divided into fixed size "slots", 8KiB in 102 length, with each slot storing a single record. Not all slots need to 103 be occupied, and they need not be occupied in a contiguous fashion. 104 The ability to clear/erase specific records allows for the formation 105 of unoccupied slots. 106 107 Slot 0 contains a backend storage header that identifies the contents 108 as ERST and also facilitates efficient access to the records. 109 Depending upon the size of the backend storage, additional slots will 110 be designated to be a part of the slot 0 header. For example, at 8KiB, 111 the slot 0 header can accommodate 1021 records. Thus a storage size 112 of 8MiB (8KiB * 1024) requires an additional slot for use by the 113 header. In this scenario, slot 0 and slot 1 form the backend storage 114 header, and records can be stored starting at slot 2. 115 116 Below is an example layout of the backend storage format (for storage 117 size less than 8MiB). The size of the storage is a multiple of 8KiB, 118 and contains N number of slots to store records. The example below 119 shows two records (in CPER format) in the backend storage, while the 120 remaining slots are empty/available. 121 122 :: 123 124 Slot Record 125 <------------------ 8KiB --------------------> 126 +--------------------------------------------+ 127 0 | storage header | 128 +--------------------------------------------+ 129 1 | empty/available | 130 +--------------------------------------------+ 131 2 | CPER | 132 +--------------------------------------------+ 133 3 | CPER | 134 +--------------------------------------------+ 135 ... | | 136 +--------------------------------------------+ 137 N | empty/available | 138 +--------------------------------------------+ 139 140 The storage header consists of some basic information and an array 141 of CPER record_id's to efficiently access records in the backend 142 storage. 143 144 All fields in the header are stored in little endian format. 145 146 :: 147 148 +--------------------------------------------+ 149 | magic | 0x0000 150 +--------------------------------------------+ 151 | record_offset | record_size | 0x0008 152 +--------------------------------------------+ 153 | record_count | reserved | version | 0x0010 154 +--------------------------------------------+ 155 | record_id[0] | 0x0018 156 +--------------------------------------------+ 157 | record_id[1] | 0x0020 158 +--------------------------------------------+ 159 | record_id[...] | 160 +--------------------------------------------+ 161 | record_id[N] | 0x1FF8 162 +--------------------------------------------+ 163 164 The 'magic' field contains the value 0x524F545354535245. 165 166 The 'record_size' field contains the value 0x2000, 8KiB. 167 168 The 'record_offset' field points to the first record_id in the array, 169 0x0018. 170 171 The 'version' field contains 0x0100, the first version. 172 173 The 'record_count' field contains the number of valid records in the 174 backend storage. 175 176 The 'record_id' array fields are the 64-bit record identifiers of the 177 CPER record in the corresponding slot. Stated differently, the 178 location of a CPER record_id in the record_id[] array provides the 179 slot index for the corresponding record in the backend storage. 180 181 Note that, for example, with a backend storage less than 8MiB, slot 0 182 contains the header, so the record_id[0] will never contain a valid 183 CPER record_id. Instead slot 1 is the first available slot and thus 184 record_id_[1] may contain a CPER. 185 186 A 'record_id' of all 0s or all 1s indicates an invalid record (ie. the 187 slot is available). 188 189 190 References 191 ---------- 192 193 [1] "Advanced Configuration and Power Interface Specification", 194 version 4.0, June 2009. 195 196 [2] "Unified Extensible Firmware Interface Specification", 197 version 2.1, October 2008. 198 199 [3] "Windows Hardware Error Architecture", specifically 200 "Error Record Persistence Mechanism".