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06dedb45 | 1 | .. SPDX-License-Identifier: GPL-2.0 |
e913ca7d | 2 | |
06dedb45 | 3 | ======================================= |
e913ca7d | 4 | efivarfs - a (U)EFI variable filesystem |
06dedb45 | 5 | ======================================= |
e913ca7d MF |
6 | |
7 | The efivarfs filesystem was created to address the shortcomings of | |
8 | using entries in sysfs to maintain EFI variables. The old sysfs EFI | |
9 | variables code only supported variables of up to 1024 bytes. This | |
10 | limitation existed in version 0.99 of the EFI specification, but was | |
11 | removed before any full releases. Since variables can now be larger | |
12 | than a single page, sysfs isn't the best interface for this. | |
13 | ||
14 | Variables can be created, deleted and modified with the efivarfs | |
15 | filesystem. | |
16 | ||
06dedb45 | 17 | efivarfs is typically mounted like this:: |
e913ca7d MF |
18 | |
19 | mount -t efivarfs none /sys/firmware/efi/efivars | |
ed8b0de5 PJ |
20 | |
21 | Due to the presence of numerous firmware bugs where removing non-standard | |
22 | UEFI variables causes the system firmware to fail to POST, efivarfs | |
23 | files that are not well-known standardized variables are created | |
24 | as immutable files. This doesn't prevent removal - "chattr -i" will work - | |
25 | but it does prevent this kind of failure from being accomplished | |
26 | accidentally. | |
2dcc51b3 FS |
27 | |
28 | .. warning :: | |
29 | When a content of an UEFI variable in /sys/firmware/efi/efivars is | |
30 | displayed, for example using "hexdump", pay attention that the first | |
31 | 4 bytes of the output represent the UEFI variable attributes, | |
32 | in little-endian format. | |
33 | ||
34 | Practically the output of each efivar is composed of: | |
35 | ||
36 | +-----------------------------------+ | |
37 | |4_bytes_of_attributes + efivar_data| | |
38 | +-----------------------------------+ | |
39 | ||
40 | *See also:* | |
41 | ||
42 | - Documentation/admin-guide/acpi/ssdt-overlays.rst | |
43 | - Documentation/ABI/stable/sysfs-firmware-efi-vars |